23. 132 Pattern.cpp

/*
132 Pattern
===========

Given an array of n integers nums, a 132 pattern is a subsequence of three integers nums[i], nums[j] and nums[k] such that i < j < k and nums[i] < nums[k] < nums[j].

Return true if there is a 132 pattern in nums, otherwise, return false.

Follow up: The O(n^2) is trivial, could you come up with the O(n logn) or the O(n) solution?


Example 1:
Input: nums = [1,2,3,4]
Output: false
Explanation: There is no 132 pattern in the sequence.

Example 2:
Input: nums = [3,1,4,2]
Output: true
Explanation: There is a 132 pattern in the sequence: [1, 4, 2].

Example 3:
Input: nums = [-1,3,2,0]
Output: true
Explanation: There are three 132 patterns in the sequence: [-1, 3, 2], [-1, 3, 0] and [-1, 2, 0]


Constraints:
n == nums.length
1 <= n <= 104
-109 <= nums[i] <= 109

*/

class Solution
{
public:
  bool find132pattern(vector<int> &nums)
  {
    stack<int> stack;
    int second = -2147483648;
    for (int i = nums.size() - 1; i >= 0; i--)
    {
      if (nums[i] < second)
        return true;
      while (stack.size() > 0 && nums[i] > stack.top())
      {
        second = stack.top();
        stack.pop();
      }

      stack.push(nums[i]);
    }
    return false;
  }
};