Description
tags: Array, Hash Table
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].
My solution
class Solution {
public:
static vector<int> twoSum(vector<int>& nums, int target) {
vector<int> result;
map<int, vector<int>> numMap;
// store nums in a map
for (auto i = 0; i < nums.size(); i++)
{
numMap[nums[i]].push_back(i);
}
for (map<int, vector<int>>::iterator it = numMap.begin(); it != numMap.end(); it++)
{
int firstVal = it->first;
int secondVal = target - firstVal;
if (firstVal == secondVal)
{
if (it->second.size() > 1)
{
result.push_back(it->second[0]);
result.push_back(it->second[1]);
return result;
}
continue;
}
else
{
if (numMap.find(secondVal) != numMap.end())
{
result.push_back(it->second[0]);
result.push_back(numMap[secondVal][0]);
return result;
}
continue;
}
}
return result;
}
};
Analysis
- Time Complexity: O(nlogn)
Space Complexity: O(n) - cons:
- Not concise enough
- Time Complexity can cut down to O(n) using
hash table
Points
-
unordered_map: use a
hash table
where the key is hashed to a slot in the table and the value is stored in a list tied to that key. O(1) finding.
map: implemented as abalanced binary search tree
(usually ared/black tree
).O(logn) finding. - further reading:
- STL Implementation
- Stackoverflow: insert vs emplace vs operator[] in c++ map
- map vs. hash_map in C++