16. 3Sum Closest

Description

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).

Solution

Three-pointers, O(n^2) time

class Solution {
    public int threeSumClosest(int[] nums, int target) {
        int res = nums[0] + nums[1] + nums[2];  // important, don't set to MAX_VALUE
        Arrays.sort(nums);
        int i = 0;
        int n = nums.length;
        
        while (i < n) {
            int j = i + 1;
            int k = n - 1;
            while (j < k) {
                int sum = nums[i] + nums[j] + nums[k];
                
                if (sum < target) {
                    ++j;
                } else if (sum > target) {
                    --k;
                } else {
                    return sum;
                }
                
                if (Math.abs(sum - target) < Math.abs(res - target)) {
                    res = sum;
                }
            }
            ++i;
        }
        
        return res;
    }
}