leetcode_001 two sum

题意分析：从数组中找出和为target的索引

解题思路：1）双重循环，时间复杂度为O(n*n）； 2）用hash表实现，时间复杂度为O(n)

第一种方法实现双重循环求解

vector<int> twoSum1(vector<int> &nums, int target){vector<int> result;int len = nums.size();if (len < 2)return result;for (int i = 0; i < len; i++){for (int j = i + 1; j < len; j++){if (nums[i] + nums[j] == target){result.push_back(i);result.push_back(j);return result; }}}return result;}

第二种方法实现hash表求解

vector<int> twoSum(vector<int> &nums, int target){// 利用hash函数实现vector<int> result;map<int, int> mp;for (int i = 0; i < nums.size(); i++){if (!mp.count(nums[i])){mp.insert(pair<int, int>(nums[i], i));}if (mp.count(target - nums[i])){int n = mp[target - nums[i]];if (n < i){result.push_back(n);result.push_back(i);return result;  }}}return result;}