1. Two Sum

原题

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

用一个map，存入target-nums[i]的值和当前下标，也就是与当前元素配对的值和下标。如果遇到map里面已经有的值就返回map里的下标和当前下标就好～

代码

//C++
class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int, int> map;
        vector<int> result;

        for (int i = 0; i < nums.size(); i++) {
            if (map.find(nums[i]) != map.end()) {
                result.push_back(map[nums[i]]);
                result.push_back(i);
                return result;
            }
            map[target - nums[i]] = i;
        }

        return result;
    }
};