1.Two Sum

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Problems with tag: Array
Problems with tag : Hash Table
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：这道题目比较简单，直接建立Hash Table来解决就可以了。方法是，先建立一个nums数组的Hash Table；然后for-loop依次遍历每个元素，看是否能够在Hash Table中找到target - nums[i]，如果能够找到，就返回i和target - nums[i]在Hash Table中对应的下标。

C++代码如下：

class Solution {public:    vector<int> twoSum(vector<int>& nums, int target) {    vector<int> res;    unordered_map<int,int> map;    for(int i = 0; i < nums.size(); i++){        int gap = target - nums[i];        if(map.find(gap) != map.end())            res = {i, map[gap]};        else            map[nums[i]] = i;    }    return res;    }};

Time: O(n).
Space: O(n).
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