leetcode 1. Two Sum

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1. Two Sum

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Question

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

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        int i,j;
        vector<int> ans;
        int n = nums.size();
        int sum;
        for(i = 0; i < n ;i++){
            for(j = i + 1;j < n ;j++){
                sum = nums[i] + nums[j];
                if(sum == target){
                    ans.push_back(i);
                    ans.push_back(j);
                    return ans;
                }
            }
        }
    }
};

考虑到题目说解是唯一的，那么用hash可求得(32ms)：

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        int i,j;
        vector<int> ans;
        map<int,int> mp;
        int n = nums.size();
        for(i = 0;i < n;i++){
            mp[ nums[i] ] = i + 1;
        }
        for(i = 0;i < n;i++){
            int left = target - nums[i];
            j = mp[ left ];
            if(j >= 1 && j != i + 1){
                ans.push_back(i);
                ans.push_back(j - 1);
                return ans;
            }
        }
    }
};