16. 最接近的三数之和

找到三数之和与target最接近，返回这三个数的和。假设答案唯一。

分析：与15.三数之和类似

同样采用 排序+双指针 方法。

public int threeSumClosest(int[] nums, int target) {
        int ans = nums[0]+nums[1]+nums[2];
        int closest = Math.abs( ans - target);
        Arrays.sort(nums);
        int n  = nums.length;
        //枚举a
        for (int first = 0; first < n; first++) {
            //twoSum
            int third = n-1;
            for (int second = first+1; second < n; second++) {
               while (second < third && (nums[first]+nums[second]+nums[third] - target)>0 ){
                   third--;
               }
               if (second == third) {
                   if (second+1<n && Math.abs(nums[first]+nums[second]+nums[second+1]-target)<closest ) {
                       ans = nums[first]+nums[second]+nums[second+1];
                       closest = Math.abs(ans-target);
                   }
               }
               else  {
                   if (Math.abs(nums[first]+nums[second]+nums[third]-target)<closest) {
                        ans = nums[first] + nums[second] + nums[third];
                        closest = Math.abs(ans-target);
                       }
                   if (third+1<n && Math.abs(nums[first]+nums[second]+nums[third+1]-target)<closest) {
                         ans = nums[first] + nums[second] + nums[third+1];
                         closest = Math.abs(ans-target);
                       }
                   }
            }

        }
        return ans;

    }