1. 两数之和

C++ 解法一：暴力法
class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        vector<int> v;
        for(int i = 0; i < nums.size(); i++) {
            for(int j = i + 1; j < nums.size(); j++) {
                if(nums[i] + nums[j] == target) {
                   v.push_back(i);
                   v.push_back(j);
                    return v;
                }
            }
        }
    }
};
c++解法二：hash_map
class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int, int> m;
        for (int i = 0; i < nums.size(); ++i) {
            if (m.count(target - nums[i])) {
                return {i, m[target - nums[i]]};
            }
            m[nums[i]] = i;
        }
        return {};
    }
};