259. 3Sum Smaller

• Difficulty: Medium

• Topics: Array, Two Pointers

• Similar Questions:

  • 3Sum

  • 3Sum Closest

  • Valid Triangle Number

  • Two Sum Less Than K

Problem:

Given an array of n integers nums and a target, find the number of index triplets i, j, k with 0 <= i < j < k < n that satisfy the condition nums[i] + nums[j] + nums[k] < target.

Example:

Input: nums = [-2,0,1,3], and target = 2
Output: 2 
Explanation: Because there are two triplets which sums are less than 2:
             [-2,0,1]
             [-2,0,3]

Follow up: Could you solve it in O(n²) runtime?

Solutions:

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

        for (int i = 0; i < nums.size() - 2; ++i) { // trap!
            int sum = target - nums[i];
            int left = i + 1;
            int right = nums.size() - 1;
            while (left < right) {
                if (nums[left] + nums[right] < sum) {
                    ret += right - left;
                    ++left;
                } else {
                    --right;
                }
            }
        }

        return ret;
    }
};