LeetCode——Increasing Triplet Subsequence

Question

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.

Solution

数学逻辑。

Code

class Solution {
public:
    bool increasingTriplet(vector<int>& nums) {
        int c1 = INT_MAX, c2 = INT_MAX;
        for (int i : nums) {
            if (i <= c1)        // 当前最小的数
                c1 = i;
            else if (i <= c2)   // 第二或者第三大的数
                c2 = i;
            else     // 如果存在一个比当前最小的大，且比第二大的数大(能够成为第二大，说明它之前有比它小的)，那么这个数前面肯定有两个比它小的
                return true;
        }
        return false;
    }
};