题目：
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, and you may not use the same element twice.

Example:

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

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

C++
解法1：暴力法
采用二重循环，暴力求解答案

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        vector<int> ans;
        for(int i = 0; i < nums.size(); i++){
            for(int j = i + 1; j < nums.size(); j++){
                if(nums[i] + nums[j] == target){
                    ans.clear();
                    ans.push_back(i);
                    ans.push_back(j);
                    return ans;
                }
            }
        }
       return ans;
    }
};

解法2：map法
新建map储存对应项数值和下标

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

java

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] ans = new int [2];
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for(int i = 0; i < nums.length; i++){
            if(map.containsKey(target - nums[i])){
                ans[0] = map.get(target - nums[i]);
                ans[1] = i;
                break;
            }
            map.put(nums[i], i);
        }
        return ans;
        
    }
}

Python

class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        map = {}
        for i in range(len(nums)):
            if target - nums[i] in map:
                return [map[target - nums[i]], i]
            map[nums[i]] = i;