611. Valid Triangle Number

# 611. Valid Triangle Number标签（空格分隔）： leetcode array medium—# 题目

Given an array consists of non-negative integers, your task is to count the number of triplets chosen from the array that can make triangles if we take them as side lengths of a triangle.

Example 1:

Input: [2,2,3,4]Output: 3Explanation:Valid combinations are: 2,3,4 (using the first 2)2,3,4 (using the second 2)2,2,3

Note:

1. The length of the given array won’t exceed 1000.
2. The integers in the given array are in the range of [0, 1000].

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思路

本题判断的规则是最小两边之和大于第三边。我们实现了一个时间复杂度为O(n2)，空间复杂度为O(1)的方法。基本思路就是先排序，再维护头尾指针遍历。

代码

class Solution {public:    int triangleNumber(vector<int>& nums) {        //step1:先排序        sort(nums.begin(),nums.end());        int count=0,num=nums.size(),l,r;        //遍历        for(int i=num-1;i>=2;i--){            l=0,r=i-1;            while(l<r)            {                if(nums[l]+nums[r]>nums[i])                {                    count+=r-l;                    r--;                }                else                    l++;            }        }        return count;    }};