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].

一刷
题解：
利用栈来保存之前的min-max intervals
If stack is empty:

• push a new Pair of num into stack

If stack is not empty:

• if num < stack.peek().min, push a new Pair of num into stack

• if num >= stack.peek().min, we first pop() out the peek element, denoted as last

  • if num < last.max, we are done, return true;

  • if num >= last.max, we merge num into last, which means last.max = num.
    Once we update last, if stack is empty, we just push back last.
    However, the crucial part is:
    If stack is not empty, the updated last might:

    • Entirely covered stack.peek(), i.e. last.min < stack.peek().min (which is always true) && last.max >= stack.peek().max, in which case we keep popping out stack.peek().
    • Form a 1-3-2 pattern, we are done ,return true

目的：保证stack里面保存的pair是无overlap的，且是按照min-value递减的。所以peek()处的min为全局最小值。

public class Solution {
    class Pair{
        int min, max;
        public Pair(int min, int max){
            this.min = min;
            this.max = max;
        }
    }
    
    public boolean find132pattern(int[] nums) {
        Stack stack = new Stack<>();
        for(int num : nums){
            if(stack.isEmpty() || numstack.peek().min){
                Pair last = stack.pop();
                if(num < last.max) return true;
                last.max = num;
                while(!stack.isEmpty() && num >= stack.peek().max) stack.pop();
                if(!stack.isEmpty() && stack.peek().min