456. 132 Pattern

Given a sequence of n integers a1, a2, ..., an, a 132 pattern is a subsequence ai, aj, ak such that i < j < k and ai < ak < aj. Design an algorithm that takes a list of n numbers as input and checks whether there is a 132 pattern in the list.

Note: n will be less than 15,000.

Example 1:

Input: [1, 2, 3, 4]

Output: False

Explanation: There is no 132 pattern in the sequence.

Example 2:

Input: [3, 1, 4, 2]

Output: True

Explanation: There is a 132 pattern in the sequence: [1, 4, 2].

Example 3:

Input: [-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].

//https://discuss.leetcode.com/topic/67881/single-pass-c-o-n-space-and-time-solution-8-lines-with-detailed-explanation

public class Solution {

public boolean find132pattern(int[] nums) {

int s3 = Integer.MIN_VALUE;

Stack<Integer> st = new Stack<Integer>();

for(int i = nums.length-1; i >= 0; i--){

if(nums[i] < s3) {

return true;

}

while(!st.empty() && nums[i] > st.peek() ) {

s3 = st.pop();

}

st.push(nums[i]);

}

return false;

}

}