611.Valid-Triangle-Number

611. Valid Triangle Number

题目地址

https://leetcode.com/problems/valid-triangle-number/

题目描述

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].

代码

Approach #1

Time: O(n^2) && Space: O(1)

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

            }
        }

        return cnt;
    }
}