leetcode 题解 || 3Sum Closest 问题

标签：二分查找   leetcode   三数求和   贪心算法   

problem：

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).

thinking：

（1）是不是很熟悉，这道题只不过是求三数和为一个特定值的变形

（2）仍然采用 贪心的思想，三指针二分查找，寻找三数和与target差值的绝对值最小的和数，这里采用了flag标记正负关系。

code :

class Solution {
public:
    int threeSumClosest(vector<int> &num, int target) {
    int index;
    bool flag=true;
    sort(num.begin(),num.end());
        if(num.at(0)+num.at(1)+num.at(2)>target)
            index=num.at(0)+num.at(1)+num.at(2)-target ;
        else
           {
                index=target-(num.at(0)+num.at(1)+num.at(2));
                flag=false;
            }

        for (int i = 0; i < num.size(); ++i)
        {

            int p = i + 1, q = num.size() - 1;

            int sum=0;

            while (p < q)
            {
                sum = num[i] + num[p] + num[q];

                if (sum == target)
                {
                    return sum;
                }//if
                else if (sum < target)  //和太小，p向后移动
                {
                    ++p;
                    if(target-sum<index)
                    {
                        index=target-sum;
                        flag=false;
                    }
                }
                else            //和过大，q向前移动
                {
                    --q;
                    if(sum-target<index)
                    {
                        index=sum-target;
                        flag=true;
                    }
                }//else
            }//while
        }//for
        if(flag)
            return index+target;
        else
            return target-index;

    }
};<strong>
</strong>

leetcode 题解 || 3Sum Closest 问题

标签：二分查找   leetcode   三数求和   贪心算法   

原文地址：http://blog.csdn.net/hustyangju/article/details/44452895