Most Asked Must-Know String + Array Interview Questions

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String-Based Questions

1. Reverse a string — without using built-ins.

2. Is it a palindrome? Let’s find out.

3. Remove duplicates from a string — efficiently.

4. First non-repeating character — who stands alone?

5. Count how many times each character appears.

  1. Flip the words in a sentence, not the letters.

7. Are two strings anagrams? Prove it.

  1. Longest substring without repeats — sliding window style.
  2. Build your own atoi — string to integer.

10. Compress strings with run-length encoding.

Array-Based Questions

26. Reverse an array in-place.

27. Find the largest and smallest element.

28. Check for duplicates in an array.

29. Remove duplicates — return only unique values.

  1. Find the missing number from 1 to N.

You Should Know:

Reverse a String in Python (Without Built-ins)

def reverse_string(s): 
return s[::-1]

Check Palindrome in Bash

!/bin/bash 
is_palindrome() { 
local str="$1" 
local len=${str} 
for (( i=0; i<len/2; i++ )); do 
if [[ "${str:i:1}" != "${str:len-i-1:1}" ]]; then 
echo "Not a palindrome" 
return 1 
fi 
done 
echo "Palindrome" 
}

Remove Duplicates from an Array in JavaScript

const removeDuplicates = (arr) => [...new Set(arr)];

Find Missing Number in Linux Shell

!/bin/bash 
find_missing() { 
arr=("$@") 
n=${arr[@]} 
total=$(( (n + 1)  (n + 2) / 2 )) 
sum=0 
for num in "${arr[@]}"; do 
sum=$((sum + num)) 
done 
echo $((total - sum)) 
}

Rotate Array in C++

include <algorithm> 
void rotate(vector<int>& nums, int k) { 
k %= nums.size(); 
reverse(nums.begin(), nums.end()); 
reverse(nums.begin(), nums.begin() + k); 
reverse(nums.begin() + k, nums.end()); 
}

Sliding Window for Longest Substring (Python)

def lengthOfLongestSubstring(s): 
char_set = set() 
left = 0 
res = 0 
for right in range(len(s)): 
while s[bash] in char_set: 
char_set.remove(s[bash]) 
left += 1 
char_set.add(s[bash]) 
res = max(res, right - left + 1) 
return res

What Undercode Say:

Mastering string and array manipulations is crucial for coding interviews. Practice these algorithms in multiple languages (Python, Bash, JavaScript, C++) to strengthen problem-solving skills. Use Linux commands like awk, sed, and `grep` for text processing. For competitive programming, optimize with bitwise operations and memoization.

Expected Output:

  • Reversed String: `dlroW olleH`
  • Palindrome Check: `”madam” → True`
  • Deduplicated Array: `[1, 2, 3, 4]`
  • Missing Number: `5`
  • Longest Substring: `”abcabcbb” → 3`

Prediction:

Future interviews will increasingly test real-time coding on collaborative platforms (e.g., CoderPad, CodeSignal). Expect more system design + DSA hybrid questions for senior roles.

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