1.Two Sum/hash map

题目描述

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].

解法

1.暴力法时间复杂度O(n²)
2.hash map+在线处理

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        map<int,int> mapp;
        for(int i=0;i<nums.size();i++){
            int diff=target-nums[i];
            if(mapp.find(diff)!=mapp.end()){
                return vector<int>{mapp[diff],i};
            }
            else mapp[nums[i]]=i;
        }
        return vector<int>{-1,-1};
    }
};