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, 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].


这个数组是无序的

java（8ms）：

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

C++（9ms）：

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