16.3Sum Closest

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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).

解法1：O(N^2)

closest转化为程序的意思就是说，需要一个var来记录每一次的sum，然后把当前的diff和之前diff来比较。
循环的时候从头开始像后循环。
C++

1

Java

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public class Solution {
    public int threeSumClosest(int[] nums, int target) {
        if (nums == null || nums.length == 0) {
            return 0;
        }
        Arrays.sort(nums);
        int closest = 0;
        for (int i = 0; i < 3 && i < nums.length; i++) {
            closest += nums[i];
        }
        if (nums.length < 3) {
            return closest;
        }
        for (int i = 0; i < nums.length - 2; i++) {
            int j =  i + 1;
            int k = nums.length - 1;
            while (j < k) {
                int sum = nums[i] + nums[j] + nums[k];
                if (Math.abs(sum - target) < Math.abs(closest - target)) {
                    closest = sum;
                    if (closest == target) return closest;
                }
                if (sum < target) {
                    j++;
                } else {
                    k--;
                }
            }
        }
        return closest;
    }
}