Program to count number of valid triangle triplets in C++

Suppose we have an array of numbers, we have to find the number of triplets chosen from the array that can make triangles if we take them as side lengths of a triangle. So if the input is like [2,2,3,4], then the result will be 3 as there are three triplets [2,3,4] using first 2, [2,3,4] using second 2, and [2,2,3].

To solve this, we will follow these steps −

ret := 0, n := size of nums, sort nums
for i in range n − 1 down to 0
- right := i − 1, left := 0
- while left < right
  - sum := nums[left] + nums[right]
  - if sum > nums[i], then increase ret by right − left, decrease right by 1, otherwise increase left by 1
return ret

Let us see the following implementation to get better understanding −

Example

Live Demo

#include <bits/stdc++.h>
using namespace std;
class Solution {
   public:
   int triangleNumber(vector<int>& nums) {
      int ret = 0;
      int n = nums.size();
      sort(nums.begin(), nums.end());
      for(int i = n − 1; i >= 0; i−−){
         int right = i − 1;
         int left = 0;
         while(left < right){
            int sum = nums[left] + nums[right];
            if(sum > nums[i]){
               ret += right − left;
               right−−;
            }else left++;
         }
      }
      return ret;
   }
};
main(){
   vector<int> v = {2,2,3,4};
   Solution ob;
   cout << (ob.triangleNumber(v));
}