leetcode 1. Two Sum

1. Two Sum

Total Accepted: 193086 Total Submissions: 899528 Difficulty: Medium

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.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

UPDATE (2016/2/13):
The return format had been changed to zero-based indices. Please read the above updated description carefully.

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最简单的思路，两层for循环，640ms

 1 class Solution {
 2 public:
 3     vector<int> twoSum(vector<int>& nums, int target) {
 4         int i,j;
 5         vector<int> ans;
 6         int n = nums.size();
 7         int sum;
 8         for(i = 0; i < n ;i++){
 9             for(j = i + 1;j < n ;j++){
10                 sum = nums[i] + nums[j];
11                 if(sum == target){
12                     ans.push_back(i);
13                     ans.push_back(j);
14                     return ans;
15                 }
16             }
17         }
18     }
19 };

考虑到题目说解是唯一的，那么用hash可求得(32ms)：

 1 class Solution {
 2 public:
 3     vector<int> twoSum(vector<int>& nums, int target) {
 4         int i,j;
 5         vector<int> ans;
 6         map<int,int> mp;
 7         int n = nums.size();
 8         for(i = 0;i < n;i++){
 9             mp[ nums[i] ] = i + 1;
10         }
11         for(i = 0;i < n;i++){
12             int left = target - nums[i];
13             j = mp[ left ];
14             if(j >= 1 && j != i + 1){
15                 ans.push_back(i);
16                 ans.push_back(j - 1);
17                 return ans;
18             }
19         }
20     }
21 };