611. Valid Triangle Number

Problem:
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].

Analysis:

2sum related problem. Duplicate is allowed. Triangle qualifies the rule that a + b > c, where c > b > a. So that we can check from array's end to start. 

Solution:

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