Search & Sorting Primer

The third part of my interview preparation focusing on searching and sorting. Find the introduction on data structures here.

  • Binary search
    • Applied on a sorted array
    • Recursive or iterative implementations
    • If there are more than one matching results (i.e. numbers are not distinct) returns a random index. To find the first one go left.
    • Examples: 9.3 in 1
  • Generic Search



  • Stable: the relative order of equal elements is preserved.
  • In-place: returns the input sorted (not a copy of it), in practice this means it requires O(1 or logn) auxiliary space.
  • Comparative: based on comparisons
    • Comparative algorithms can't do better than nlogn on average or worst cases.
    • All of the following are comparative except counting-radix-bucket.
  • General method: insertion, exchange, selection, merging, etc.
  • Adaptability: Whether or not the presorted-ness of the input affects the running time.
Name Time (best-worst) Space Stable? In-place
Insertion n-n^2 O(1) yes yes
Selection n^2 O(1) no yes
Heapsort nlogk-nlogn O(1) no yes
Mergesort nlogn O(n) yes no
Quicksort nlogn-n^2 O(logn) no (usually)  
Counting sort n k - (doesn't make sense for ints) yes


  • Insertion: splits sorted and unsorted part of the input, implementation looks back in the sorted part for every element. Can be used as the base case for other divide n conquer algos (merge, quicksort or bucket sort)
  • Selection: Implementation looks ahead for the next minimum
    • Heapsort: selection sort with a (max or min) heap
  • Quicksort: divide n conquer, partition method. best theoretical explanation here, worst case when the pivot divides in very small and very big lists (recurrence T(n) = T(n-1) + O(n) and quadratic time).
    • Found 2 similar implementations (more here)
      1. Choose pivot, swap it to the end (or just choose last), keep an index of first larger than pivot element and run through the elements swaping with a smaller element. At the end swap the pivot with the larger element. Repeat for left and right side e.g. here or 3
      2. Choose pivot, swap left with right elements until the pointers cross, repeat for left and right list, see 1
    • Introsort: Quicksort + Heapsort
    • Practical example: 18.6 in 1
  • Mergesort: divide 'n' conquer, merging method.
    • Timsort: mergesort + insertion
  • Counting3
    • Bucket3: uniform distribution and ordered hash, hash + insertion sort in the bucket.
    • Radix: get max, counting sort per digit


  • Small n or almost sorted? insertion (worst case on reversely sorted array)
  • Guaranteed nlogn worst case or k-sorted array? heapsort
  • Stability is a must, we sort a linked list (or other slow random access data) or data does not fit in main memory: mergesort
  • Fastest average case: quicksort
  • Reverse input: shell sort or heap?
  • Values within a range (possibly with many duplicates): counting sort. Range too big? Radix or bucket sort (provided uniform distribution of input)
  • Want to minimise swaps? Selection

Cool visualisation

Java implementation details

  • Quicksort needs logn space.