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MEDIUM

3Sum Closest

Given an integer array nums of length n and an integer target, find three integers in nums such that the sum is closest to target.

Return the sum of the three integers. You may assume that each input would have exactly one solution.

Example

Input:

nums = [-1,2,1,-4], target = 1

Output:

2

Explanation:
The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

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Constraints

• 3 ≤ nums.length ≤ 500
• -1000 ≤ nums[i] ≤ 1000
• -10⁴ ≤ target ≤ 10⁴

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Solution: Two Pointers after Sorting

• Time Complexity: O(n²), where n is the length of the array.
• Space Complexity: O(1), as only a constant amount of extra space is used.

C++

class Solution {
public:
    int threeSumClosest(vector<int>& nums, int target) {
        sort(nums.begin(), nums.end());
        int n = nums.size();
        int closest = nums[0] + nums[1] + nums[2];
        for (int i = 0; i < n - 2; ++i) {
            int left = i + 1, right = n - 1;
            while (left < right) {
                int sum = nums[i] + nums[left] + nums[right];
                if (abs(sum - target) < abs(closest - target))
                    closest = sum;
                if (sum < target)
                    ++left;
                else
                    --right;
            }
        }
        return closest;
    }
};