3Sum Smaller in C++

Suppose we have an array of n integers called nums and we also have a target, we have to find the number of index triplets (i, j, k) here i, j, k all are in range 0 to n - 1 and that satisfy the condition nums[i] + nums[j] + nums[k] < target.

So, if the input is like nums = [-2,0,1,3], and target = 2, then the output will be 2, as there are two triplets which sums are less than 2: [-2,0,1] and [-2,0,3].

To solve this, we will follow these steps −

• ret := 0

• sort the array a

• n := size of a

• for initialize i := 0, when i < n - 2, update (increase i by 1), do −

  • left := i + 1, right := n - 1

  • while left < right, do −

    • sum := a[i] + a[left] + a[right]

    • if sum < t, then −

      • ret := ret + right - left

      • (increase left by 1)

    • Otherwise

      • (decrease right by 1)

• return ret

Example 

Let us see the following implementation to get better understanding −

 Live Demo

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

Input

[-2,0,1,3] 2

Output

2