✅ Core Coding Interview Questions With Answers - Part 6 [Python Code] 🖥️

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51. How do you reverse a string?

s = "hello"
# Method 1: Slicing
reversed_s = s[::-1]  # "olleh"

# Method 2: Two Pointers (In-place logic)
chars = list(s)
left, right = 0, len(chars) - 1
while left < right:
    chars[left], chars[right] = chars[right], chars[left]
    left += 1
    right -= 1
reversed_s = ''.join(chars)

52. How do you check if a string is a palindrome?

def is_palindrome(s):
    # Clean string: lowercase and remove spaces
    s = s.lower().replace(" ", "")
    
    # Method 1: Slicing
    return s == s[::-1]

    # Method 2: Two Pointers
    left, right = 0, len(s) - 1
    while left < right:
        if s[left] != s[right]:
            return False
        left += 1
        right -= 1
    return True

53. How do you find duplicates in an array?

arr = [1, 2, 2, 3]
seen = set()
dups = set()

for num in arr:
    if num in seen:
        dups.add(num)
    seen.add(num)

print(list(dups))  # Output: [2]

54. How do you find the missing number in a range from 1 to n?

arr = [1, 2, 4] # Missing 3
n = len(arr) + 1 # Should be 4 elements total
expected_sum = n * (n + 1) // 2
actual_sum = sum(arr)

missing_number = expected_sum - actual_sum  # 3

55. How do you merge two sorted arrays?

arr1, arr2 = [1, 3], [2, 4]
i, j = 0, 0
result = []

while i < len(arr1) and j < len(arr2):
    if arr1[i] < arr2[j]:
        result.append(arr1[i])
        i += 1
    else:
        result.append(arr2[j])
        j += 1

# Append remaining elements
result.extend(arr1[i:])
result.extend(arr2[j:])

56. How do you find the nth Fibonacci number?

def fib(n):
    if n <= 1: 
        return n
    a, b = 0, 1
    for _ in range(2, n + 1):
        a, b = b, a + b
    return b

print(fib(6))  # Output: 8

57. How do you compute factorial? (Recursion vs Memoization)

# Simple Recursion
def fact(n):
    if n <= 1: return 1
    return n * fact(n - 1)

# Recursive with Memoization (Optimization)
memo = {}
def fact_memo(n):
    if n in memo: return memo[n]
    if n <= 1: return 1
    memo[n] = n * fact_memo(n - 1)
    return memo[n]

print(fact(5))  # Output: 120

58. How do you remove duplicates from a sorted array in-place?

arr = [1, 1, 2, 2, 3]
if not arr: return 0

slow = 0
for fast in range(1, len(arr)):
    if arr[fast] != arr[slow]:
        slow += 1
        arr[slow] = arr[fast]

# Resulting array up to 'slow + 1' index
print(arr[:slow + 1]) # Output: [1, 2, 3]

59. How do you solve the Two Sum problem?

nums, target = [2, 7, 11, 15], 9
mapping = {}

for i, num in enumerate(nums):
    complement = target - num
    if complement in mapping:
        print([mapping[complement], i]) # Output: [0, 1]
    mapping[num] = i

60. Interview tip you must remember

- Code Cleanly: Use meaningful variable names (e.g., current_sum instead of s).
- Test Immediately: Verbally walk through your code with a small test case before the interviewer asks you to.
- Discuss Optimization: Always mention Time and Space Complexity. Say: *"This is O(n) time and O(n) space. We could optimize space by..."*

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