3Sum Closest | Leetcode

Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

    For example, given array S = {-1 2 1 -4}, and target = 1.

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

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Solution: Sort the input array. Iterate through the array. Keep track of the closest sum found so far. For each element i, go through elements i+1 to the last element in such a way that we try to get closer to target by advancing either the left or right pointer.

public class Solution {
    public int threeSumClosest(int[] nums, int target) {
        // If you want to preserve the input, simply clone the array first before sorting.
        Arrays.sort(nums);
        
        int closestSum = nums[0] + nums[1] + nums[2];
        int closestDiff = Math.abs(closestSum - target);
        
        for (int i = 0; i < nums.length - 2; i++) {
            int n = nums[i];
            int left = i + 1;
            int right = nums.length - 1;
            
            while (left < right) {
                int sum = n + nums[left] + nums[right];
                if (sum == target) return sum;
                
                int diff = Math.abs(target - sum);
                if (diff < closestDiff) {
                    closestSum = sum;
                    closestDiff = diff;
                }
                
                if (sum < target) {
                    left++;
                } else {
                    right--;
                }
            }
        }
        
        return closestSum;
    }
}