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+#+title: Leetcode doc
+#+author: Aryadev Chavali
+#+description: Description
+#+date: 2024-04-29
+
+* Prelude
+#+begin_src c++ :session c++
+test
+#+end_src
+* Arrays
+** DONE (#217) Contains duplicate
+If there are any duplicates, then when the list is sorted they would
+be consecutive.
+#+begin_src c++ :tangle 217.cpp :session c++
+#include <bits/stdc++.h>
+
+using namespace std;
+
+bool containsDuplicate(vector<int> &nums)
+{
+  std::sort(nums.begin(), nums.end());
+  const size_t n = nums.size() - 1;
+  for (const size_t i = 0; i < n; ++i)
+    if (nums[i] == nums[i + 1])
+      return true;
+  return false;
+}
+#+end_src
+** DONE (#242) Valid anagram
+If two things are anagrams of each other then they are equivalent
+disregarding ordering.
+
+*** Sorting the strings
+Sorting the two strings would force the same
+order for both strings.
+
+#+begin_src c++ :tangle 242-sort.cpp
+#include <bits/stdc++.h>
+using namespace std;
+bool isAnagram(string s, string t)
+{
+  std::sort(s.begin(), s.end());
+  std::sort(t.begin(), t.end());
+  return s == t;
+}
+#+end_src
+*** Simple table
+#+begin_src c++ :tangle 242-table.cpp
+#include <bits/stdc++.h>
+using namespace std;
+bool isAnagram(string s, string t)
+{
+  if (s.size() != t.size())
+    return false;
+  int table[26] = {0};
+  for (size_t i = 0; i < s.size(); ++i)
+  {
+    table[s[i] - 'a'] += 1;
+    table[t[i] - 'a'] -= 1;
+  }
+
+  return std::all_of(table, table + 26, [](int i) { return i == 0; });
+}
+#+end_src
+** DONE (#1) Two sum
+Given a target find two elements which sum to it.
+#+begin_src c++ :tangle 1.cpp
+#include <bits/stdc++.h>
+using namespace std;
+vector<int> twoSum(vector<int> &nums, int target)
+{
+  unordered_map<int, int> diffs;
+  for (int i = 0; i < nums.size(); ++i)
+  {
+    if (diffs.find(target - nums[i]) != diffs.end())
+      return {diffs[target-nums[i]], i};
+    else
+      diffs[nums[i]] = i;
+  }
+  return {};
+}
+#+end_src
+** DONE (#49) Group anagrams
+#+begin_src c++ :tangle 49.cpp
+#include <bits/stdc++.h>
+using namespace std;
+vector<vector<string>> groupAnagrams(vector<string> &strs)
+{
+  unordered_map<string, vector<string>> map;
+  for (auto str : strs)
+  {
+    string s = str;
+    sort(s.begin(), s.end());
+    map[s].push_back(str);
+  }
+
+  vector<vector<string>> ret;
+  for (const auto &x : map)
+    ret.push_back(x.second);
+  return ret;
+}
+#+end_src
+** DONE (#347) Top k frequent elements
+Get the top k most frequency elements in an array.  Frequencies imply
+hash tables.
+*** Sorting the map
+Once you've generated the map, sort it by frequency (then by number)
+then take the top k off it.
+#+begin_src c++ :tangle 347-sort.cpp
+#include <bits/stdc++.h>
+using namespace std;
+vector<int> topKFrequent(vector<int> &nums, int k)
+{
+  unordered_map<int, int> map;
+  for (auto num : nums)
+    map[num] += 1;
+
+  vector<pair<int, int>> vec(map.begin(), map.end());
+  sort(vec.begin(), vec.end(),
+       [](pair<int, int> a, pair<int, int> b)
+       {
+         if (a.second == b.second)
+           return a.first > b.first;
+         return a.second > b.second;
+       });
+
+  vector<int> topk;
+  for (size_t i = 0; i < k; ++i)
+    topk.push_back(vec[i].first);
+  return topk;
+}
+#+end_src
+*** Using a queue
+Instead of sorting the map, we can use a priority queue.  Remember
+that an item in a priority queue has two members: the priority and the
+value.  The priority sorts the members.
+
+Hence, if we generate the map then place them all in a priority queue,
+we just need to get the k highest priority items to finish the
+problem.
+#+begin_src c++ :tangle 347-queue.cpp
+#include <bits/stdc++.h>
+using namespace std;
+
+vector<int> topKFrequent(vector<int> &nums, int k)
+{
+  unordered_map<int, int> map;
+  for (const auto num : nums)
+    map[num] += 1;
+
+  priority_queue<pair<int, int>> queue;
+  for (const auto &pair : map)
+    // priority=frequency, value=value
+    queue.push({pair.second, pair.first});
+
+  vector<int> topk;
+  for (size_t i = 0; i < k; ++i)
+  {
+    topk.push_back(pq.top().second);
+    pq.pop();
+  }
+  return topk;
+}
+#+end_src
+** DONE (#238) Product of array except self
+Return an array where at each index i the element is simply the
+product of all elements except the input@i.  You cannot use division
+and it has to be O(n).
+
+Mathematically this is asking for the product [nums[x] | x from 0 to
+n] / nums[i].  We can split this into [x from 0 to i - 1] and [x from
+i to n] and consider them.  These products work linearly as we iterate
+i: [x from 0 to i - 1] * i is the product [x from 0 to i].  Same the
+other way.  So we can iterate once to produce the products before i
+and once for the products after i while only doing a set once!
+
+#+begin_src c++ :tangle 238.cpp
+#include <bits/stdc++.h>
+
+using namespace std;
+
+vector<int> productexceptself(vector<int> &nums)
+{
+  const int n = nums.size();
+  vector<int> out(n, 0);
+  out[0] = 1;
+  for (int i = 0; i < n - 1; ++i)
+    out[i + 1] = out[i] * nums[i];
+  int postfix = 1;
+  for (int i = n - 1; i >= 0; --i)
+    out[i] *= postfix;
+  postfix *= nums[i];
+  return out;
+}
+#+end_src
+** DONE (#36) Valid sudoku
+Determine if a 9x9 grid is a valid sudoku.
+
+You can use a set of hashsets (where membership testing is O(1)) to
+just check if you've seen a number before.  In this case, since we
+know the exact input data and the properties we want we can use
+boolean tables to speed up the process.
+
+I also use a little maths technique to produce a correct ordering of
+the boxes via integer division.
+#+begin_src c++ :tangle 36.cpp
+#include <bits/stdc++.h>
+using namespace std;
+
+constexpr size_t box_count(const size_t row, const size_t col)
+{
+  return (3 * (row / 3)) + (col / 3);
+}
+
+bool isValidSudoku(vector<vector<char>> &board)
+{
+  bool row_digits[9][9] = { 0 };
+  bool col_digits[9][9] = { 0 };
+  bool box_digits[9][9] = { 0 };
+
+  for (size_t i = 0; i < 9; ++i)
+  {
+    for (size_t j = 0; j < 9; ++j) {
+      if (board[i][j] == '.')
+        continue;
+      const auto val = board[i][j] - '0' - 1;
+      if (box_digits[box_count(i, j)][val] || col_digits[j][val] ||
+          row_digits[i][val])
+        return false;
+      box_digits[box_count(i, j)][val] = true;
+      col_digits[j][val] = true;
+      row_digits[i][val] = true;
+    }
+  }
+
+  return true;
+}
+#+end_src
+** DONE (#128) Longest consecutive sequence
+
+#+begin_src c++ :tangle 128.cpp
+#include <bits/stdc++.h>
+using namespace std;
+
+int longestConsecutive(vector<int> &nums)
+{
+  unordered_set<int> s{nums.begin(), nums.end()};
+  size_t size = 0;
+  for (auto num : nums)
+  {
+    if (s.find(num - 1) != s.end())
+      continue;
+    size_t seq = 1;
+    for (size_t x = num; s.find(x + 1) != s.end(); ++x, ++seq)
+      continue;
+    size = size < seq ? seq : size;
+  }
+  return size;
+}
+#+end_src
+* Stack
+** DONE (#20) Valid Parentheses
+#+begin_src c++ :tangle 20.cpp
+#include <bits/stdc++.h>
+using namespace std;
+
+#define check_top(COUNTER, WANTED) !(COUNTER.size() != 0 && COUNTER.top() == WANTED)
+
+bool isValid(string s)
+{
+  stack<char> counter;
+  for (const auto x : s)
+  {
+    switch (x) {
+    case '(':
+    case '{':
+    case '[':
+      counter.push(x);
+      break;
+    case ')':
+      if (check_top(counter, '('))
+        return false;
+      counter.pop();
+      break;
+    case '}':
+      if (check_top(counter, '{'))
+        return false;
+      counter.pop();
+      break;
+    case ']':
+      if (check_top(counter, '['))
+        return false;
+      counter.pop();
+      break;
+    }
+  }
+  return counter.size() == 0;
+}
+#+end_src
+** DONE (#155) Min Stack
+Make a minimal implementation of a stack which does all its main
+functions in O(1) (what?!).
+
+In exchange for O(n) memory we can implement most of them.
+Unfortunately all the high performance implementations also use stacks
+(what's the point!).
+#+begin_src c++ :tangle 155.cpp
+#include <bits/stdc++.h>
+
+using namespace std;
+
+class MinStack
+{
+private:
+  stack<int> ordered;
+  size_t ptr;
+  vector<int> data;
+public:
+  MinStack(): data{}, ptr{0}
+  {
+  }
+
+  void push(int val)
+  {
+    if (ptr == data.size())
+      data.push_back(val);
+    else
+      data[ptr] = val;
+
+    if (ordered.size() == 0 || ordered.top() >= val)
+      ordered.push(val);
+
+    ++ptr;
+  }
+
+  void pop()
+  {
+    --ptr;
+    if (data[ptr] == ordered.top())
+      ordered.pop();
+  }
+
+  int top()
+  {
+    return data[ptr - 1];
+  }
+
+  int getMin()
+  {
+    return ordered.top();
+  }
+};
+#+end_src
+** DONE (#150) Evaluate RPN
+You're given an RPN expression in tokens e.g. ["2", "3", "+"], run it.
+
+Encountered an issue with C/C++'s semantics when it comes prefix
+decrement: it doesn't have an effect until after the statement so the
+other side of the assignment still considers ~ptr~ to be the same
+value.  Bit interesting, good to note.
+#+begin_src c++ :tangle 150.cpp
+#include <bits/stdc++.h>
+using namespace std;
+
+int evalRPN(vector<string>& tokens)
+{
+  vector<int> stack{};
+  size_t ptr = 0;
+  for (const auto &token : tokens)
+  {
+    if (token == "+")
+      stack[(--ptr) - 1] += stack[ptr - 1];
+    else if (token == "-")
+      stack[(--ptr) - 1] -= stack[ptr - 1];
+    else if (token == "*")
+      stack[(--ptr) - 1] *= stack[ptr - 1];
+    else if (token == "/")
+      stack[(--ptr) - 1] /= stack[ptr - 1];
+    else
+    {
+      int n = stoi(token);
+      if (stack.size() <= ptr)
+        stack.push_back(n);
+      else
+        stack[ptr] = n;
+      ++ptr;
+    }
+  }
+  return stack[0];
+}
+#+end_src
+** DONE (#22) Generate Parentheses
+Given n, generate all possible well formed combinations of
+parentheses.
+
+*** Naive solution
+I generate all 2^n bit strings (where n = 2 * number of pairs) then
+filer all the invalid bit strings.
+#+begin_src c++ :tangle 22-naive.cpp
+#include <bits/stdc++.h>
+
+using namespace std;
+
+unordered_map<int, unordered_set<bitset<16>>> memo;
+
+bool is_valid(size_t size, bitset<16> n)
+{
+  stack<bool> s;
+  for (size_t i = 0; i < size; ++i)
+  {
+    if (n[i])
+      s.push(true);
+    else
+    {
+      if (s.size() == 0)
+        return false;
+      s.pop();
+    }
+  }
+  return s.size() == 0;
+}
+
+string bits_to_string(size_t size, bitset<16> n)
+{
+  stringstream ss;
+  for (size_t i = 0; i < size; ++i)
+    ss << (n[i] ? "(" : ")");
+  return ss.str();
+}
+
+unordered_set<bitset<16>> recursive(int n)
+{
+  if (memo.find(n) != memo.end())
+    return memo[n];
+  auto prev = recursive(n - 1);
+  unordered_set<bitset<16>> res{};
+  for (const auto &map : prev)
+  {
+    bitset<16> has_1 = map;
+    has_1[n - 1] = true;
+    res.insert(has_1);
+    res.insert(map);
+  }
+  memo[n] = res;
+  return res;
+}
+
+vector<string> generateParenthesis(int n)
+{
+  memo[1].insert(0);
+  memo[1].insert(1);
+  unordered_set<bitset<16>> sets = recursive(2 * n);
+  vector<string> strings;
+  for (const auto &set : sets)
+    if (is_valid(2 * n, set))
+      strings.push_back(bits_to_string(2 * n, set));
+  return strings;
+}
+#+end_src
+*** Backtracking
+Instead of generating all bit strings and then filtering out, let's
+just build the strings bit by bit?  Here we use a queue (though we
+could use recursion) to generate bigger and bigger paren strings.  We
+pop off the queue and check three things: if the string is the right
+size then push it onto the vector.  If the string has insufficient
+number of open braces, add an open brace.  If the string has
+insufficient closed braces in comparison to open braces add a close
+brace.
+
+Notice the heuristic: we generate both combinatorial paths for the
+string (add an open brace or add a closed brace) but only generate the
+second one based on a condition on the first.  As we do these
+simultaneously we can generate the two possible next steps given an
+input string.
+#+begin_src c++ :tangle 22-backtrack.cpp
+#include <bits/stdc++.h>
+
+using namespace std;
+
+struct Work
+{
+  int open_braces, close_braces;
+  string str;
+};
+
+vector<string> generateParenthesis(int n)
+{
+  vector<string> results;
+  queue<Work> queue;
+  queue.push({0, 0, ""});
+  while (queue.size() != 0)
+  {
+    auto res = queue.front();
+    queue.pop();
+    if (res.str.size() == (size_t)2 * n)
+      results.push_back(res.str);
+    if (res.open_braces < n)
+      queue.push({res.open_braces + 1, res.close_braces, res.str + "("});
+    if (res.close_braces < res.open_braces)
+      queue.push({res.open_braces, res.close_braces + 1, res.str + ")"});
+  }
+  return results;
+}
+#+end_src
+** TODO (#739) Daily temperatures
+Given an array of temperatures t[i], generate an array /a/ where a[i]
+is the number of days it takes to surpass t[i].  If there is no
+temperature which surpasses t[i], a[i] = 0.
+
+#+begin_src c++ :tangle 739.cpp :comments link
+#include <bits/stdc++.h>
+
+using namespace std;
+
+vector<int> dailyTemperatures(vector<int> &temperatures)
+{
+  stack<int> stack;
+  vector<int> days{(int)temperatures.size()};
+  for (size_t i = 0; i < temperatures.size(); ++i)
+  {
+    while (!stack.empty() && temperatures[stack.top()] < temperatures[i])
+    {
+      int prev   = stack.top();
+      days[prev] = i - prev;
+      stack.pop();
+    }
+    stack.push(i);
+  }
+  return days;
+}
+#+end_src