1. Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

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

Example:
Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].
UPDATE (2016/2/13):
The return format had been changed to zero-based indices. Please read the above updated description carefully.

思路：开始想到的是sort，然后用首尾双指针的方法，但是这个要返回index而不是值，sort完后index就变了。

class Solution {public:    vector<int> twoSum(vector<int>& nums, int target) {        vector<int> ans;        if(nums.empty()) return ans;        unordered_map<int,int> data;        for(int i=0; i < nums.size(); i++)        {            if(data.find(nums[i]) == data.end())            {                data[nums[i]] = i;             }            int j = target - nums[i];            if(data.find(j)!=data.end())            {                if(data[j] != i)                {                    ans.push_back(i);                    ans.push_back(data[j]);                    return ans;                }            }        }        return ans;    }};