Leetcode:611. Valid Triangle Number

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: 3
Explanation:
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].

題意：給出數列nums，求隨機取3個數能夠組成的三角形的個數。
滿足三角形：兩條小邊的和大於大邊，因此首先進行排序。
重點:三重循環確定三邊，最大邊用二分法確定（當時面試時死活都沒有想到的優化。。。。。。。）。
代碼如下：

class Solution {
    public int triangleNumber(int[] nums) {
        Arrays.sort(nums);
        int n=nums.length;
        int count=0;
        for(int i=0;i<n-2;i++)
        {
            for(int j=i+1;j<n-1;j++)
            {
                int l=j;
                int r=n;
                while(l+1<r){
                    int mid=(l+r)/2;
                    if(nums[i]+nums[j]<=nums[mid])
                        r=mid;
                    else
                        l=mid;
                }
                count+=l-j;
            }
        }
        return count;
    }
}