611. Valid Triangle Number

• Difficulty: Medium

• Topics: Array

• Similar Questions:

  • 3Sum Smaller

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].

Solutions:

class Solution {
public:
    int triangleNumber(vector<int>& nums) {
        if (nums.size() < 3)    return 0;
        sort(nums.begin(), nums.end());
        int count = 0;

        for (int i = 0; i < nums.size() - 2; ++i) {
            int a = nums[i];
            int bIndex = i + 1;
            int cIndex = i + 2;

            for (; cIndex < nums.size(); ++cIndex) {
                while (bIndex < cIndex && a + nums[bIndex] <= nums[cIndex]) {
                    ++bIndex;
                }

                if (cIndex > bIndex) {
                    count += (cIndex - bIndex);
                }
            }
        }

        return count;
    }
};