Leetcode 1:two Sum

Title Description:

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

The function twosum should return indices of the numbers such that they add up to the target, where index1 must is Les S than Index2. Please note that your returned answers (both Index1 and INDEX2) is not zero-based.

You may assume this each input would has exactly one solution.

Input:numbers={2, 7, one, A, target=9
Output:index1=1, index2=2

Thinking:

1. First did not notice examining, thought that the array is sorted, then uses two pointers, one after the other, until the target and the pointer is found

    Vector<int> twosum (vector<int> &numbers, int target) {        int start = 0;        int tail = numbers.size ()-1;        vector<int> result;        while (Start<tail) {            if (Numbers[start] + Numbers[tail] <target) {                start + +;            } else if (Numbers[start] + Numbers[tail] > target) {                tail--;            } else{                result.push_back (start);                Result.push_back (tail);                return result;}}    }

After submission, find runtime Error

2. Then consider the method of violence, Time complexity O (n^2)

    Vector<int> twosum (vector<int> &numbers, int target) {        vector<int> result;        for (int i =0;i<numbers.size (), i++) {for            (int j=i+1;j<numbers.size (); j + +) {                if (Numbers[i] + numbers[j) = = Target) {                    result.push_back (i+1);                    Result.push_back (j+1);                    return result;}}}    

　　

Discover time Limit exceeded.

3. The search for a faster method, the online reference, found the need to Hashtable, using the STL map to achieve.

    Vector<int> twosum (vector<int> &numbers, int target) {        vector<int> result;        Map<int,int> Map_res;        for (int i =0;i<numbers.size (); i++) {            int temp = target-numbers[i];            if (Map_res[temp]) {                result.push_back (map_res[temp]+1);                Result.push_back (i+1);            }            map_res[numbers[i]]=i;        }        return result;    }

4. Found the result is still wrong, Runtime error, the error example is [0,4,3,0], 0, found that the original value of Map_res[0] corresponds to 0, changed the method, using Map_res.count (temp), finally AC

    Vector<int> twosum (vector<int> &numbers, int target) {        vector<int> result;        Map<int,int> Map_res;        for (int i =0;i<numbers.size (); i++) {            int temp = target-numbers[i];            if (Map_res.count (temp)) {                result.push_back (map_res[temp]+1);                Result.push_back (i+1);            }            map_res[numbers[i]]=i;        }        return result;    }

　　

Leetcode 1:two Sum