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].
Solution 1: use unordered_map to store <num,index>, find whether target-num exists in the map(O(1))
1 class Solution { 2 public: 3 vector<int> twoSum(vector<int>& nums, int target) { 4 unordered_map<int,int> m; 5 vector<int> res; 6 for (int i=0;i<nums.size();i++){ 7 m[nums[i]]=i; 8 } 9 for (int i=0; i<nums.size();i++){ 10 int x=target-nums[i]; 11 if(m.count(x)&&m[x]!=i){ 12 res.push_back(i); 13 res.push_back(m[x]); 14 break; 15 } 16 } 17 return res; 18 } 19 };