[Leetcode] 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).

3Sum的变种题，实际上就是枚举所有3Sum的情况，首先将数组排序，然后枚举第一个元素，之后用两个指针指向这个元素后面的数组部分，当三个数的和大于target的时候，移动右指针，因为这个时候再移动左指针只会让数变得更大，没有意义，小于target的时候，移动左指针，等于的时候直接返回就可以了。

    public int threeSumClosest(int[] num, int target) {
        Arrays.sort(num);
        int best = num[0] + num[1] + num[2];
        for(int i = 0; i < num.length-2; i++) {
            if(i != 0 && num[i-1] == num[i]) continue;
            int l = i+1, r = num.length-1;
            while(l < r) {
                int sum = num[i] + num[l] + num[r];
                if(Math.abs(sum - target) < Math.abs(best - target)) {
                    best = sum;
                }
                if(sum < target) {
                    l++;
                    while(l < num.length && num[l] == num[l-1])
                        l++;
                }
                else if(sum > target) {
                    r--;
                    while(r >= 0 && num[r] == num[r+1])
                        r--;
                }
                else return sum;
            }
        }
        
        return best;
    }