LeetCode - Two Sum

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution.

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

使用hash表记录每个数值出现的位置

构建hash表需要O(n)的时间和空间

使用STL的map查找需要O(log(n)

class Solution {public:    vector<int> twoSum(vector<int> &numbers, int target) {        vector<int> ret;        map<int,int> m;        for(int i=0;i<numbers.size();i++){            m[numbers[i]]=i;        }        for(int i=0;i<numbers.size();i++){            if(m.find(target-numbers[i])!=m.end()){                int ind=m[target-numbers[i]];                if(ind!=i){                    ret.push_back(i+1);                    ret.push_back(m[target-numbers[i]]+1);                    break;                }            }        }        return ret;    }};