456. 132 Pattern

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Problem

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.

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].

Code

class Solution {
    public boolean find132pattern(int[] nums) {  
        int numk = Integer.MIN_VALUE;
        // save num > numk -> numj
        Stack<Integer> stack = new Stack<>();

        for (int i = nums.length - 1; i >= 0; i--) {
            if(nums[i] < numk) return true;
            
            while(!stack.isEmpty() && nums[i] > stack.peek()) {
                numk = stack.pop();
            }
            
            stack.push(nums[i]);
        }
        
        return false;
    }
}