Top 20 Hash Table Interview Questions and Answers (2026)

1. Introduction to Hash Tables

Hash tables enable O(1) average lookup and insertion. Below are 20 essential questions with complete solutions in C++, Java, Python, and JavaScript.

2. Core Hash Table Questions

Q1. Two Sum

cpp
vector<int> twoSum(vector<int>& nums, int target) {
    unordered_map<int, int> mp;
    for (int i = 0; i < nums.size(); i++) {
        int comp = target - nums[i];
        if (mp.count(comp)) return {mp[comp], i};
        mp[nums[i]] = i;
    }
    return {};
}

java
public int[] twoSum(int[] nums, int target) {
    Map<Integer, Integer> mp = new HashMap<>();
    for (int i = 0; i < nums.length; i++) {
        int comp = target - nums[i];
        if (mp.containsKey(comp)) return new int[]{mp.get(comp), i};
        mp.put(nums[i], i);
    }
    return new int[]{};
}

python
def twoSum(nums: list, target: int) -> list:
    mp = {}
    for i, num in enumerate(nums):
        comp = target - num
        if comp in mp: return [mp[comp], i]
        mp[num] = i
    return []

javascript
function twoSum(nums, target) {
    let mp = new Map();
    for (let i = 0; i < nums.length; i++) {
        let comp = target - nums[i];
        if (mp.has(comp)) return [mp.get(comp), i];
        mp.set(nums[i], i);
    }
    return [];
}

Time Complexity: O(n) | Space Complexity: O(n)

3. Summary Table

Practice all hash table problems on DSAMaster's practice platform.