[leetcode] 334. Increasing Triplet Subsequence 解题报告

题目链接：https://leetcode.com/problems/increasing-triplet-subsequence/

Given an unsorted array return whether an increasing subsequence of length 3 exists or not in the array.

Formally the function should:

Return true if there exists  i, j, k 
such that  arr[i] <  arr[j] <  arr[k] given 0 ≤  i <  j <  k ≤  n-1 else return false.

Your algorithm should run in O(n) time complexity and O(1) space complexity.

Examples:
Given [1, 2, 3, 4, 5],
return true.

Given [5, 4, 3, 2, 1],
return false.

思路：本题可以用最长升序子序列的方法来做，但是有点小题大做了。还可以维护一个当前的长度为2的升序序列(小的值叫small, 大的叫big)，如果碰到比第二个值大的说明可以找到升序的三个值。并且在过程中不断更新small和big的值，使得他们最小。

代码如下：

class Solution {
public:
    bool increasingTriplet(vector<int>& nums) {
        int small = INT_MAX, big = INT_MAX;
        for(auto val: nums)
        {
            if(val <= small) small = val;
            else if(val <= big) big = val;
            else return true;
        }
        return false;
    }
};

参考：https://leetcode.com/discuss/86891/concise-java-solution-with-comments