3SUM Closest

Question

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).

Analysis

This problem can be reduced to 2SUM problem after selecting the first number. The time complexity is O(n*n), space complexity is O(1).

Solution

public int threeSumClosest(int[] nums, int target) {
        Arrays.sort(nums);
        int result=nums[0]+nums[1]+nums[2];
        for(int i=0;i<nums.length-2;i++){
            int lo=i+1, hi=nums.length-1;
            while(lo<hi){
                int sum=nums[lo]+nums[hi]+nums[i];
                if(sum==target) return target;
                if(Math.abs(result-target)>Math.abs(sum-target)) result=sum;
                if(sum<target) lo++;
                else           hi--;
            }
        }
        return result;
    }