Merge Sort Implementation in C# - StackOverflow Exception

Question

everyone! I am trying to implement the merge sort algorithm in C#. And I have a StackOverflow Exception with my code. I'm trying to figure out where is the problem and I can't solve it.

using System;

class MainClass {
  
  static void merge(int[] arr, int leftIndex, int endLeftIndex, int endRightIndex){
    // leftArr [leftIndex ... endLeftIndex]
    // rightArr [endLeftIndex + 1 ...  endRightIndex]
    int leftArrLength = endLeftIndex - leftIndex + 1;
    int rightArrLength = endRightIndex - endLeftIndex;
    
    int[] leftArr = new int[leftArrLength], rightArr = new int[rightArrLength];

    // Copying the data from the parent Array to the leftArr & rightArr
    for(int i = 0; i < leftArrLength; i++)
      leftArr[i] = arr[leftIndex + i];
    for(int j = 0; j < rightArrLength; j++)
      rightArr[j] = arr[endLeftIndex + j + 1];

    // the start index of the array that will be merged
    int mergedArrIndex = leftIndex;

    // set two start pointers for both left and right arrays
    int leftIndexMerge = 0, rightIndexMerge = 0;

    // the merging operation
    while(leftIndexMerge < leftArrLength && rightIndexMerge < rightArrLength){
      if(leftArr[leftIndexMerge] < rightArr[rightIndexMerge])
        arr[mergedArrIndex++] = leftArr[leftIndexMerge++];
      else
        arr[mergedArrIndex++] = rightArr[rightIndexMerge++];
    }
    
    // copying the rest elements
    while(leftIndexMerge < leftArrLength)
      arr[mergedArrIndex++] = leftArr[leftIndexMerge++];
    
    while(rightIndexMerge < rightArrLength)
      arr[mergedArrIndex++] = rightArr[rightIndexMerge++];
  }
  
  static void mergeSort(int[] arr, int leftIndex, int endRightIndex){
    if(leftIndex < endRightIndex){
      // leftArr [leftIndex ... m]
      // rightArr [m + 1 ...  endRightIndex]
      int m = leftIndex + (endRightIndex - 1) / 2;
      mergeSort(arr, leftIndex, m);
      mergeSort(arr, m + 1, endRightIndex);
      merge(arr, leftIndex, m, endRightIndex);
    }
  }
  public static void Main (string[] args) {
    int[] arr = {1,2,5,10,1,2,3,4,4};
    mergeSort(arr, 0, arr.Length);
    foreach(int i in arr) Console.WriteLine(i);
  }
}

View the code in repl.it: https://repl.it/@MohamedMarzouk/MergeSortImplementationInCSharp

Answer 1

You had 2 problems

The first which caused your stack overflow

// int m = leftIndex + (endRightIndex - 1) / 2;
// should be
int m = leftIndex + (endRightIndex - leftIndex) / 2;

Looking at the source, you just mistook l for a 1

and the following which would have given you an index out of range error

// mergeSort(arr, 0, arr.Length);
// should be
mergeSort(arr, 0, arr.Length-1);

No excuses there :)