611 Valid Triangle Number

611. Valid Triangle Number

1. Question

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

2. Implementation

(1) Two Pointer

class Solution {
    public int triangleNumber(int[] nums) {
        if (nums == null || nums.length <= 2) {
            return 0;
        }

        Arrays.sort(nums);
        int count = 0;

        for (int i = 0; i < nums.length - 2; i++) {
            int k = i + 2;
            for (int j = i + 1; nums[i] != 0 && j < nums.length - 1; j++) {
                while (k < nums.length && nums[i] + nums[j] > nums[k]) {
                    k++;
                }
                count += k - j - 1;
            }
        }
        return count;
    }
}

3. Time & Space Complexity

Two Pointer: 时间复杂度O(n^2), 空间复杂度O(1),因为sorting需要空间