[LeetCode] 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:
The length of the given array won't exceed 1000.
The integers in the given array are in the range of [0, 1000].

Solution

class Solution {
    public int triangleNumber(int[] nums) {
        //for triangle: a+b > c
        if (nums == null || nums.length < 3) return 0;
        
        Arrays.sort(nums);
        int count = 0;
        for (int i = 2; i < nums.length; i++) {
            int l = 0, r = i-1;
            while (l < r) {
                if (nums[l] + nums[r] > nums[i]) {
                    count += r-l;
                    r--;
                }
                else l++;
            }
        }
        return count;
    }
}