# LeetCode 答案

记录一个C++小白的刷题辛酸泪. 以及感谢我的猴不辞辛劳地为我讲解各种数据结构和提供解答.

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.
Examples:

Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Language: cpp

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

知识点:

1. unordered_map: unordered_map和map类似，都是存储的key-value的值，可以通过key快速索引到value. map的内部数据结构为红黑树，map中的元素是按照二叉搜索树存储的, 因此map中的元素都是有序的, 时间复杂度为O(logN). unordered_map采用哈希表, 时间复杂度为O(1).
2. return {} ；