【LeetCode】Two Sum

题目描述：

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution.

Input: numbers={2, 7, 11, 15}, target=9

Output: index1=1, index2=2
思路：暴力居然能过，可见数据不大，跑了52ms，优化了下，先排序，然后再从两端开始遍历，复杂度只有排序的O（nlogn），跑了12ms。
代码：

class Solution {
public:
vector<int> twoSum(vector<int> &numbers, int target) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
vector<int> res;

struct node{
int val,index;
node(int v,int i){
val=v;
index=i;
}
bool operator<(const node &x) const
{
return val<x.val;
}

};
vector<node> buf;

for(int i=0; i<numbers.size(); i++)
{
buf.push_back(node(numbers[i],i+1));
}

sort(buf.begin(),buf.end());

int i=0,j=numbers.size()-1;
while(i<j)
{
if(buf[i].val+buf[j].val == target)
{
res.push_back(buf[i].index);
res.push_back(buf[j].index);
break;
}
else if(buf[i].val+buf[j].val < target)
{
i++;
}
else j--;
}

sort(res.begin(),res.end());
return res;
}

};