# 最接近的三数之和
<p>给定一个包括&nbsp;<em>n</em> 个整数的数组&nbsp;<code>nums</code><em>&nbsp;</em>和 一个目标值&nbsp;<code>target</code>。找出&nbsp;<code>nums</code><em>&nbsp;</em>中的三个整数，使得它们的和与&nbsp;<code>target</code>&nbsp;最接近。返回这三个数的和。假定每组输入只存在唯一答案。</p><p>&nbsp;</p><p><strong>示例：</strong></p><pre><strong>输入：</strong>nums = [-1,2,1,-4], target = 1<strong><br />输出：</strong>2<strong><br />解释：</strong>与 target 最接近的和是 2 (-1 + 2 + 1 = 2) 。</pre><p>&nbsp;</p><p><strong>提示：</strong></p><ul>	<li><code>3 &lt;= nums.length &lt;= 10^3</code></li>	<li><code>-10^3&nbsp;&lt;= nums[i]&nbsp;&lt;= 10^3</code></li>	<li><code>-10^4&nbsp;&lt;= target&nbsp;&lt;= 10^4</code></li></ul>
<p>以下错误的选项是？</p>
## aop
### before
```cpp
#include <bits/stdc++.h>

using namespace std;
```
### after
```cpp
int main()
{
    Solution sol;
    int arr[] = {-1, 2, 1, -4};
    int target = 1;
    int length1 = sizeof(arr) / sizeof(arr[0]);
    vector<int> nums(arr, arr + length1);
    int res;
    res = sol.threeSumClosest(nums, target);
    cout << res;
    return 0;
}
```

## 答案
```cpp
class Solution
{
public:
    int threeSumClosest(vector<int> &nums, int target)
    {
        if (nums.size() < 3)
            ;
        runtime_error("No Solution");
        sort(nums.begin(), nums.end());
        int res = nums[0] + nums[1] + nums[2];
        int dValue = abs(target - res);
        int l, r, sum;
        for (int i = 0; i < nums.size() - 2; ++i)
        {
            l = i + 1;
            r = nums.size() - 1;
            if (3 * nums[i] > target)
                break;
            while (l < r)
            {
                sum = nums[i] + nums[l] + nums[r];
                if (sum < target)
                    --l;
                else if (sum > target)
                    ++r;
                else
                    return target;
                if (abs(target - sum) < dValue)
                {
                    res = sum;
                    dValue = abs(target - sum);
                }
            }
        }
        return res;
    }
};
```
## 选项

### A
```cpp
class Solution
{
public:
    int threeSumClosest(vector<int> &nums, int target)
    {
        int l = nums.size();
        int best = nums[0] + nums[1] + nums[2];
        for (int i = 0; i < l - 2; i++)
        {
            for (int j = i + 1; j < l - 1; j++)
            {
                for (int k = j + 1; k < l; k++)
                {
                    int t = nums[i] + nums[j] + nums[k];
                    if (abs(t - target) < abs(best - target))
                    {
                        best = t;
                    }
                }
            }
        }
        return best;
    }
};
```

### B
```cpp
class Solution
{
public:
    int threeSumClosest(vector<int> &nums, int target)
    {
        sort(nums.begin(), nums.end());
        if (nums.size() < 3)
            return {};
        int closest = target + 10000000;
        for (int i = 0; i < nums.size() - 2; i++)
        {
            int fix = nums[i];
            if (fix > target && abs(fix + nums[i + 1] + nums[i + 2] - target) > abs(closest - target))
                break;

            for (int j = i + 1; j < nums.size() - 1; j++)
            {
                for (int k = j + 1; k < nums.size(); k++)
                {
                    int temp = abs(target - fix - nums[j] - nums[k]);
                    if (temp < abs(closest - target))
                        closest = fix + nums[j] + nums[k];
                }
            }
        }
        return closest;
    }
};
```

### C
```cpp
class Solution
{
public:
    int threeSumClosest(vector<int> &nums, int target)
    {
        int len = nums.size();
        int min = target + 1000000;
        sort(nums.begin(), nums.end());
        for (int i = 0; i < len; i++)
        {
            int fir = nums[i];
            int r = len - 1;
            int l = i + 1;
            if (i > 0 && fir == nums[i - 1])
                continue;
            if (l >= r)
                break;
            while (l < r)
            {
                if (nums[l] + nums[r] + fir == target)
                    return target;
                if (nums[l] + nums[r] < target - fir)
                {
                    int temp = target - fir - nums[r] - nums[l];
                    if (temp < abs(min - target))
                        min = nums[l] + nums[r] + fir;
                    l++;
                }
                else if (nums[l] + nums[r] > target - fir)
                {
                    int temp = nums[l] + nums[r] + fir - target;
                    if (temp < abs(min - target))
                        min = nums[l] + nums[r] + fir;
                    r--;
                }
            }
        }
        return min;
    }
};
```