学步园

用了和3Sum差不多一样的思路，二分查找。关键要剪枝，但是却在剪枝那里犯了很多错误。

然后原来有一个更加快的思路O(n^2).

#include<iostream>  
#include <vector>  
#include <algorithm>  
#include <math.h>
using namespace std;


class Solution {
public:

	int threeSumClosest(vector<int> &num, int target) {

		int minDis = 1 << 30;
		int closetSum = -1<<30;

		sort(num.begin(), num.end());
		for (int i = 0; i < num.size() - 2; i++)
		{
			for (int j = i + 1; j < num.size() - 1; j++)
			{

				int twoSum = num[i] + num[j];
				int value = target - twoSum;

				int tmpCloVal = searchValue(num, j + 1, num.size() - 1, value);

				if (abs(target - twoSum - tmpCloVal) < minDis)
				{
					closetSum = twoSum + tmpCloVal;
					minDis = abs(target - closetSum);
				}

				if ( num[j+1]>0 && num[i]+num[j] > target)
					break;
			}

			if (num[i+1]>0 && num[i] > target)
				break;
		}

		return closetSum;
	}

	int searchValue(vector<int> num, int left, int right, int value)
	{
		const int l = left, r = right;
		int closetVal;
		int m;

		if (value > num[right])
			return num[right];
		else if (value < num[left])
			return num[left];

		while (left <= right){
			m = (left + right) / 2;

			if (num[m] <= value && num[m + 1] >= value)
			{
				break;
			}

			else if (num[m] < value)
			{
				left = m + 1;
			}
			else
			{
				right = m - 1;
			}
		}

		closetVal = num[m];

		if (abs(closetVal-value) > abs(num[m + 1] - value))
		{
			closetVal = num[m+1];
		}
		if (abs(closetVal - value) > abs(num[m-1]-value))
		{
			closetVal = num[m - 1];
		}

		return closetVal;
	}

};