DSA & Algorithms · Full Mock 2 – Mock Test

Question 1 C strings Beginner Practice Code executed

What does this print?

C main.c

#include <stdio.h>
#include <string.h>

int main(void) {
    char s[] = "hello";
    printf("%zu %zu\n", sizeof(s), strlen(s));
    return 0;
}

5 5 6 6 5 6 6 5

Question 2 Character codes Intermediate Practice Code executed

This converts a numeric string to an integer by hand. What does it print?

C main.c

#include <stdio.h>

int main(void) {
    const char *s = "472";
    int n = 0;
    for (int i = 0; s[i]; i++) {
        n = n * 10 + (s[i] - '0');
    }
    printf("%d\n", n + 1);
    return 0;
}

Your answer

Question 3 2D arrays Advanced Practice Code executed

What does this print?

Python main.py

1
2
3

grid = [[0] * 3] * 3
grid[0][0] = 1
print(grid)

[[1, 0, 0], [0, 0, 0], [0, 0, 0]] TypeError [[1, 0, 0], [1, 0, 0], [1, 0, 0]] [[1, 1, 1], [0, 0, 0], [0, 0, 0]]

Question 4 String DP Advanced Practice Answer computed

What is the length of the longest common substring (contiguous) of ABABC and BABCA?

Your answer

Question 5 Grid graphs Intermediate Practice Answer computed

Moving up, down, left or right through open cells (0) only, what is the fewest number of moves from the top-left to the bottom-right cell? (1 = wall)

Maze

	0	1	2	3
0	0	0	0	1
1	1	1	0	1
2	0	0	0	0
3	1	0	1	0

Your answer

Question 6 Shortest paths Advanced Practice Answer computed

A signal leaves node 2. Directed travel times: 2→1 (1), 2→3 (1), 3→4 (1), 1→4 (4). How long until all four nodes have received it?

Network

Your answer

Question 7 Interval scheduling Beginner Practice Answer computed

Train arrivals are 9:00, 9:40, 9:50, 11:00, 15:00, 18:00 and departures 9:10, 12:00, 11:20, 11:30, 19:00, 20:00 (same order). What is the minimum number of platforms?

Your answer

Question 8 Two-pass greedy Intermediate Practice Answer computed

Children with ratings [1, 0, 2] stand in a row. Each gets at least one candy, and a child with a higher rating than a neighbour gets more than that neighbour. What is the minimum number of candies?

Your answer

Question 9 Counting Beginner Practice Code executed

What does this program print?

C++ main.cpp

#include <iostream>
#include <sstream>
#include <string>
#include <unordered_map>

int main() {
    std::istringstream in("the cat saw the hat on the mat");
    std::unordered_map<std::string, int> freq;
    std::string w;
    while (in >> w) freq[w]++;
    std::cout << freq["the"] * 10 + freq.size() << '\n';
}

Your answer

Question 10 Hash maps Intermediate Practice Code executed

Does the array contain two equal values at most k indices apart? What does this print?

Python main.py

def near_dup(a, k):
    last = {}
    for i, x in enumerate(a):
        if x in last and i - last[x] <= k:
            return True
        last[x] = i
    return False

print(near_dup([1, 2, 3, 1, 2, 3], 2), near_dup([1, 0, 1, 1], 1))

True True False False True False False True

Question 11 Hash maps Advanced Practice Code executed

What does this print?

C++ main.cpp

#include <iostream>
#include <string>
#include <unordered_map>

int main() {
    std::unordered_map<std::string, int> m;
    if (m["a"] > 0) std::cout << "has a\n";
    if (m.count("b")) std::cout << "has b\n";
    if (m.find("c") != m.end()) std::cout << "has c\n";
    std::cout << m.size() << '\n';
}

Your answer

Question 12 Heap operations Beginner Practice Answer computed

The maximum is removed from this max-heap (last element moved to the root, then sifted down). Which heap results?

Max-heap (stored as [50, 30, 40, 10, 20])

Question 13 Simulation Intermediate Practice Answer computed

Repeatedly smash the two heaviest stones: if they differ, the difference goes back. Starting with [2, 7, 4, 1, 8, 1], what weight is left at the end (0 if none)?

Your answer

Question 14 Slow and fast pointers Beginner Practice Code executed

The slow pointer moves one step and the fast pointer two. What does this print?

Python main.py

class Node:
    def __init__(self, val, next=None):
        self.val = val
        self.next = next

def build(vals):
    head = None
    for v in reversed(vals):
        head = Node(v, head)
    return head

def show(head):
    out = []
    while head:
        out.append(head.val)
        head = head.next
    print(*out)

head = build([10, 20, 30, 40, 50])
slow = fast = head
while fast and fast.next:
    slow = slow.next
    fast = fast.next.next
print(slow.val)

Your answer

Question 15 Cycle detection Intermediate Practice Answer computed

How many nodes are in the cycle of this list?

Linked list

head123456↺ back to 3

Your answer

Question 16 Strings Beginner Practice Code executed

What does this print?

Python main.py

def rev(s):
    if s == "":
        return ""
    return rev(s[1:]) + s[0]

print(rev("stack"))

stack kcats tacks kstac

Question 17 Grid recursion Intermediate Practice Answer computed

A recursive flood fill starts at the top-left cell and spreads to 4-directionally adjacent cells containing 1. How many cells does it fill?

Grid

	0	1	2	3
0	1	1	0	0
1	0	1	0	1
2	1	1	0	1
3	0	0	1	1

Your answer

Question 18 Binary search variants Intermediate Practice Code executed

This binary search finds a peak (an element larger than its neighbours). Which index does it return?

Python main.py

a = [1, 2, 1, 3, 5, 6, 4]
lo, hi = 0, len(a) - 1
while lo < hi:
    mid = (lo + hi) // 2
    if a[mid] < a[mid + 1]:
        lo = mid + 1
    else:
        hi = mid
print(lo)

Your answer

Question 19 Binary search Advanced Practice Code executed

What does this print?

C++ main.cpp

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {1, 2, 4, 4, 4, 7};
    auto lo = std::lower_bound(v.begin(), v.end(), 4) - v.begin();
    auto hi = std::upper_bound(v.begin(), v.end(), 4) - v.begin();
    std::cout << lo << ' ' << hi << ' ' << hi - lo << '\n';
}

2 5 2 3 5 2 2 4 2 2 5 3

Question 20 BST insertion Beginner Practice Answer computed

Key 5 is inserted into this BST. Which tree results?

Binary search tree

Question 21 BST search Intermediate Practice Answer computed

In this BST, what are floor(9) (largest key ≤ 9) and ceil(12) (smallest key ≥ 12)?

Binary search tree

710 1013 814 813

Question 22 Tries Advanced Practice Answer computed

The words tea, ten, to, inn and in are inserted into an empty trie. How many nodes are there, not counting the root?

Your answer

Question 23 Space complexity Beginner Practice

What is the space complexity of this function, counting the call stack?

Python main.py

def sum_to(n):
    if n == 0:
        return 0
    return n + sum_to(n - 1)

O(1) O(n²) O(log n) O(n)

Question 24 Loops Intermediate Practice

This checks whether a sorted array has two numbers adding up to target. What is its time complexity?

Python main.py

def has_pair(a, target):
    i, j = 0, len(a) - 1
    while i < j:
        s = a[i] + a[j]
        if s == target:
            return True
        if s < target:
            i += 1
        else:
            j -= 1
    return False

O(log n) O(n) O(n log n) O(n²)

Question 25 Notation Advanced Practice One or more answers

Which statements are true? Select all that apply.

n log n = O(n) n² = O(n³) log(n!) = Θ(n log n) 2ⁿ⁺¹ = O(2ⁿ) 2²ⁿ = O(2ⁿ)

Question 26 Foundations Beginner Practice

For "number of ways to climb n stairs with 1 or 2 steps", what is the best definition of dp[i]?

The number of steps taken so far The largest stair reachable in i moves Whether stair i can be reached The number of distinct ways to reach stair i

Question 27 Subsequences Intermediate Practice

The O(n log n) LIS algorithm keeps an array tails. What does tails[k] hold?

The smallest possible tail value of an increasing subsequence of length k + 1 seen so far The number of subsequences of length k The actual LIS The largest element seen so far

Question 28 Basics Beginner Practice

What is the minimum number of edges in a connected undirected graph with 10 vertices?

Your answer

Question 29 Greedy foundations Advanced Practice

What is an "exchange argument" in proving a greedy algorithm correct?

Show the greedy algorithm is fast Take any optimal solution and show it can be transformed step by step into the greedy one without getting worse Swap the input order and rerun Run the greedy and DP algorithms and compare results

Question 30 Array layout Advanced Practice

A heap has 31 elements (a perfect tree). How many nodes have height exactly 1 (one level above the leaves)?

Your answer

Question 31 Deep copy Advanced Practice

Each node also has a random pointer to any node. How can the list be deep-copied in O(n) time with O(1) extra space (besides the copy)?

Copy values into an array and rebuild Insert each copy right after its original, set copy.random = orig.random.next, then separate the two lists It needs O(n) extra space – a hash map is unavoidable Use recursion on random pointers

Question 32 Permutations Advanced Practice

What is the time complexity of generating and storing all permutations of n distinct elements?

O(n · n!) O(n!) / n O(2ⁿ) O(n²)

Question 33 Binary search Beginner Practice

About how many comparisons does binary search need in the worst case on 1,000,000 sorted elements?

About 1000 About 500,000 About 20 About 1,000,000

Question 34 Circular queues Beginner Practice

Why is an array-based queue usually implemented as a circular buffer?

To make the queue a stack To keep the elements sorted To allow access to the middle element So slots freed at the front can be reused by wrapping the indices with modulo, without shifting elements

Question 35 Priority queues Intermediate Practice

How does a priority queue differ from a normal queue?

It only stores integers It removes the element with the highest priority (e.g. smallest value), not the oldest; usually a heap with O(log n) operations It removes the newest element first It is a queue implemented with two stacks

Question 36 Deque Advanced Practice

The sliding-window-maximum algorithm keeps a deque of indices with decreasing values. What is its total time and extra space for n elements and window k?

O(n·k) time, O(1) space O(n) time, O(k) space O(n) time, O(n²) space O(n log k) time, O(k) space

DSA & Algorithms · Full Mock 2

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