16. 3Sum Closest

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：Two pointers

思路：Two pointers 并 keep comparing and updating closest
Time Complexity: O(N^2) Space Complexity: O(1)

Solution Code：

class Solution {
    public int threeSumClosest(int[] nums, int target) {
        int min_diff = Integer.MAX_VALUE;
        int result = 0;
        if(nums == null || nums.length == 0) return result;
        Arrays.sort(nums);
        
        for(int i = 0; i < nums.length - 2; i++) {
            int target2num = target - nums[i];
            
            // two pointers
            int start = i + 1, end = nums.length - 1;
            while(start < end) {
                int sum2num = nums[start] + nums[end];
                int diff = Math.abs(sum2num - target2num);
                if(diff < min_diff) {
                    min_diff = diff;
                    result = nums[i] + sum2num; 
                }
                if(sum2num <= target2num) {
                    start++;
                }
                else {
                    end--;
                }
            }
        }
        return result;
    }
}