TwoSum(算法分析课week1)

TwoSum

题目来源：https://leetcode.com/problemset/algorithms/

• 问题描述
• 解题思路
• 代码实现

---

问题描述

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, and you may not use the same element twice.

Example:
Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

解题思路

这个问题的实现很简单，利用两层循环就可以实现，但是要注意一个数字不可以使用两次，这就要求第二层循环变量不能等于第一层循环变量。而且每个输入只有一个解，所以得到一个解就可以跳出循环。所以我所使用的算法的时间复杂度为O(n^2).

代码实现

class Solution {public:    vector<int> twoSum(vector<int>& nums, int target) {        vector<int> twoSum;        vector<int>::iterator it;        int i = 0;        int flag = 0;        for (it=nums.begin();it!=nums.end();it++, i++) {            vector<int>::iterator it1;            it1 = it;            it1++;            int j = i;            j++;            for (it1; it1 != nums.end(); it1++, j++) {                if (*it + *it1 == target) {                    twoSum.push_back(i);                    twoSum.push_back(j);                    flag = 1;                    break;                }            }             if (flag == 1) break;        }        return twoSum;     }};