LeetCode 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).

题意：给出一个数组及一个目标数，求数组中三个数加起来与目标数最接近的数

思路：与LeetCode 4Sum(backtrace+剪枝)有些相似

代码如下：

class Solution {    private int kSum(int[] nums, int start, int k,int cur_sum, int target)    {        int len = nums.length;        if (start >= len || 0 == k) return 0;        if (2 == k)        {            int end = len - 1;            int ans = Integer.MAX_VALUE;            while (start < end)            {                int sum = nums[start] + nums[end];                if (sum + cur_sum == target)                {                    while (start < end && nums[start] == nums[start + 1]) start++;                    start++;                    while (start < end && nums[end] == nums[end - 1]) --end;                    --end;                    ans = target;                    return ans;                }                else if (sum + cur_sum < target)                {                    while (start < end && nums[start] == nums[start + 1]) start++;                    start++;                    if (Integer.MAX_VALUE == ans)                    {                        ans = cur_sum + sum;                    }                    else if (Math.abs(cur_sum + sum - target) < Math.abs(ans - target))                    {                        ans = cur_sum + sum;                    }                }                else                {                    while (start < end && nums[end] == nums[end - 1]) end--;                    end--;                    if (Integer.MAX_VALUE == ans)                    {                        ans = cur_sum + sum;                    }                    else if (Math.abs(cur_sum + sum - target) < Math.abs(ans - target))                    {                        ans = cur_sum + sum;                    }                }            }            return ans;        }        int ans = Integer.MAX_VALUE;        for (int i = start; i <= (len - k); i++)        {            if (i > start && nums[i] == nums[i - 1]) continue;            int tmp = kSum(nums, i + 1, k - 1, cur_sum + nums[i], target);            if (Integer.MAX_VALUE  == ans)            {                ans = tmp;            }            else if (Math.abs(tmp - target) < Math.abs(ans - target))            {                ans = tmp;            }        }        return ans;    }    public int threeSumClosest(int[] nums, int target)    {        Arrays.sort(nums);        int ans = kSum(nums, 0, 3, 0, target);        return ans;    }}

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