Most Common Programming Questions in the Interviews about data structures

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Linked Lists

You must be able to produce simple clean linked list implementation quickly.

  • Implement Insert and Delete for
    • singly-linked linked lists
    • sorted linked lists
    • circular linked lists

    int Insert(node** head, int data)
    int Delete(node** head, int deleteMe)

  • Split a linked lists given a pivot value
    void Split(node* head, int pivot, node** lt, node** gt)
  • Find if a linked lists has a cycle in it. Now do it without marking nodes.
  • Find the middle of a linked lists. Now do it while only going through the list once. (same solution as finding cycles)


  • Reverse words in a string (words are separated by one or more spaces). Now do it in-places. By far the most popular string question!
  • Reverse a string
  • Strip whitespaces from a string in-place
    void StripWhitespace(char* szStr)
  • Remove duplicate chars from a strings (“AAA BBB” -> “A B”)
    int RemoveDups(char* szStr)
  • Find the first non-repeating characters in a string:(“ABCA” -> B )
    int FindFirstUnique(char* szStr)
  • More Advanced Topics:
    • You may be asked about using Unicode string. What the interviewer is usually looking for is:
      • each character will be two bytes (so, for example, char lookup table you may have allocated needs to be expanded from 256 to 256 * 256 = 65536 elements)
      • that you would need to use wide char types (wchar_t instead of char)
      • that you would need to use wide string functions (like wprintf instead of printf)
    • Guarding against being passed invalid string pointers or non nul-terminated strings (using walking through a string and catching memory exceptions

Binary Trees

  • Implement the following functions for a binary trees:
    • Insert
    • PrintInOrder
    • PrintPreOrder
    • PrintPostOrder
  • Implement a non-recursive PrintInOrder


  • You are given an array with integer between 1 and 1,000,000. One integer is in the array twice. How can you determine which one? Can you think of a way to do it using little extra memory.
  • You are given an array with integers between 1 and 1,000,000. One integer is missing. How can you determine which one? Can you think of a way to do it while iterating through the array only once. Is overflow a problem in the solutions? Why not?
  • Returns the largest sum of contiguous integer in the array
    Example: if the input is (-10, 2, 3, -2, 0, 5, -15), the largest sum is 8
    int GetLargestContiguousSum(int* anData, int len)
  • Implement Shuffle given an array containing a deck of cards and the number of cards. Now make it O(n).
  • Return the sum two largest integers in an array
    int SumTwoLargest(int* anData, int size)
  • Sum n largest integers in an array of integers where every integer is between 0 and 9
    int SumNLargest(int* anData, int size, int n)


  • Implement a Queue class in C++ (which data structure to use internally? why? how to notify of errors?)


  • Count the number of set bits in a byte/int32
  • Difference between heap and stack? Write a function to figure out if stack grows up or down.
  • SQL query to select some rows out of a table (only because I had SQL on the resume)
  • Open a file as securely as possible (assume the user is hostile — list all the nasty things that could happen and checks you would have to do to)
  • Implement a function to return a ratio from a double (ie 0.25 -> 1/4). The function will also take a tolerance so if toleran ce is .01 then FindRatio(.24, .01) -> 1/4
    int FindRatio(double val, double tolerance, int& numerator, int& denominator)


  • You have 2 supposedly unbreakable light bulbs and a 100-floor building. Using fewest possible drops, determine how much of an impact this type of light bulb can withstand. (i.e. it can withstand a drop from 17th floor, but breaks from the 18th).
    Note that the ever-popular binary search will give you a worst case of 50 drops. You should be able to do it with under 20.
  • There are n gas stations positioned along a circular road. Each has a limited supply of gas. You can only drive clockwise around the road. You start with zero gas. Knowing how much gas you need to get from each gas station to the next and how much gas you can get at each station, design an algorithm to find the gas station you need to start at to get all the way around the circle.
  • Out of 10 coins, one weighs less then the others. You have a scale.
    • How can you determine which one weighs less in 3 weighs?
    • Now how would you do it if you didn’t know if the odd coin weighs less or more?
  • What is the next line in the following sequence:
    Answer: it's 1211 and the next is 111221

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  1. DUFO 2017-03-17 Reply

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