1 两数之和

https://leetcode.cn/leetbook/read/top-interview-questions-easy/x2jrse/

https://leetcode.cn/problems/two-sum/description/

cost:3m11s

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

• 时间复杂度：O(n)
• 空间复杂度：O(n)