2 Sum

2 Sum
Description:
Given an array of integers, find two numbers such that they add up to a special 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.
Example
numbers=[2,7,11,15], target = 9
return [1,2]
Note
You may assue that each input would have exactly one solution
Chanllenge
Either of the following solutions are acceptable:

• O(1) Space, O(nlogn) Time
• O(n) Space, O(n) Time

思路：双指针法。双指针法在链表结构和数组结构中运用。

代码如下：

#include <cstdio>
#include <vector>
#include <algorithm>

using namespace std;
int const maxn = 100;
int a[maxn];

void twoSum(int front, int end, int target){
    if (a[front]+a[end]>target){
        twoSum(front, end-1, target);
    }else if (a[front]+a[end]<target){
        twoSum(front+1, end, target);
    }else{
        vector<int> result;
        result.clear();
        result.push_back(front);
        result.push_back(end);
        printf("%d %d\n", result[0], result[1]);
        return;
    }
}

int main(){
    int T;
    int i=0;
    scanf(" %d", &T);
    int cnt = T;
    while (T--){
        scanf(" %d", a+i);
        i++;
    }
    sort(a, a+cnt);

    int target;
    scanf(" %d", &target);

    twoSum(0, cnt-1,target);

    return 0;
}

时间复杂度和空间复杂度分析：

• 时间复杂度
  • 排序用时O(nlogn)　　　　双指针移动用时O(n)------------------>   O(nlogn+n)------------->O(nlogn)
• 空间复杂度O(1)