LeetCode

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.

找到三个数使得满足a[i] < a[j] < a[k] && i < j < k

思维题吧?

设置一个small和big，如果小于small就替换small，如果大于small小于big，就替换big。否则，大于这两个数，说明条件成立，返回true。

class Solution {public:    bool increasingTriplet(vector<int>& nums) {        if (nums.size() < 3) return false;        int a1 = INT_MAX, a2 = INT_MAX;        for (auto x: nums) {            if (x <= a1) a1 = x;            else if (x <= a2) a2 = x;            else return true;        }        return false;    }};