leetcode 1. Two Sum

相关问题

1. Two Sum
15. 3Sum
16. 3Sum Closest
18. 4Sum
454. 4Sum II

Discription

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] + nums1 = 2 + 7 = 9,
return [0, 1].

思路

哈希表
遍历过程中使用哈希表记录每个出现的元素。如果其“互补”元素已经出现，则构成一组解。

时间复杂度：$O(n)$
空间复杂度：$O(n)$

代码

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        vector<int> res;

        unordered_map<int, int> hash;
        for (int i = 0; i < nums.size(); i++)
        {
            int val = target - nums[i];
            if (hash.find(val) == hash.end())
                hash[nums[i]] = i;
            else
            {
                res.push_back(i);
                res.push_back(hash[val]);
            }
        }

        return res;
    }
};