Merge Two Sorted Arrays Without Extra Space

// C Code to Merge two sorted arrays a[] and b[] without 
// extra space using insert of insertion sort

#include <stdio.h>

void mergeArrays(int a[], int b[], int n, int m) {
    
    // Traverse b[] starting from the last element
    for (int i = m - 1; i >= 0; i--) {
      
        // If b[i] is smaller than the largest element of a[]
        if (a[n - 1] > b[i]) {
          
            // Place b[i] in the correct position in a[], 
            // and move last element of a[] to b[]
            int last = a[n - 1];
            int j = n - 2;
            while (j >= 0 && a[j] > b[i]) {
                a[j + 1] = a[j];
                j--;
            }
            a[j + 1] = b[i];
            b[i] = last;
        }
    }
}

int main() {
    int a[] = {1, 5, 9, 10, 15, 20};
    int b[] = {2, 3, 8, 13};
    int n = sizeof(a) / sizeof(a[0]);
    int m = sizeof(b) / sizeof(b[0]);

    mergeArrays(a, b, n, m);
    for (int i = 0; i < n; i++)
        printf("%d ", a[i]);
    printf("\n");
    for (int i = 0; i < m; i++)
        printf("%d ", b[i]);

    return 0;
}

// C program to merge two sorted arrays a[] and b[]
// without extra space using n-th smallest number

#include <stdio.h>
#include <limits.h>

int compare(const void *a, const void *b) {
    return (*(int *)a - *(int *)b);
}

// Find m-th smallest element
// Do a binary search in a[] to find the right index idx
// in a[] such that all combined elements in a[idx..m-1]
// and b[m-idx...n-1] are greater than or equal to all
// the remaining elements (a[0..idx-1] and b[m-idx-1..n-1])
// in both the arrays.
int kthSmallest(int *a, int n, int *b, int m, int k) {
    int lo = 0, hi = n, idx = 0;
    while (lo <= hi) {
        int mid1 = (lo + hi) / 2;
        int mid2 = k - mid1;

        // We don't have mid2 elements in b[], so pick more
        // elements from a[]
        if (mid2 > m) {
            lo = mid1 + 1;
            continue;
        }

        // Find elements to the left and right of partition in a[]
        int l1 = (mid1 == 0 ? INT_MIN : a[mid1 - 1]);
        int r1 = (mid1 == n ? INT_MAX : a[mid1]);

        // Find elements to the left and right of partition in b[]
        int l2 = (mid2 == 0 ? INT_MIN : b[mid2 - 1]);
        int r2 = (mid2 == m ? INT_MAX : b[mid2]);

        // If it is a valid partition
        if (l1 <= r2 && l2 <= r1) {
            idx = mid1;
            break;
        }

        // Check if we need to take lesser elements from a[]
        if (l1 > r2)
            hi = mid1 - 1;

        // Check if we need to take more elements from a[]
        else
            lo = mid1 + 1;
    }
    return idx;
}

void mergeArrays(int *a, int n, int *b, int m) {
    int idx = kthSmallest(a, n, b, m, n);

    // Move all smaller elements in a[]
    for (int i = idx; i < n; i++) {
        int temp = a[i];
        a[i] = b[i - idx];
        b[i - idx] = temp;
    }

    // Sort both a[] and b[]
    qsort(a, n, sizeof(int), compare);
    qsort(b, m, sizeof(int), compare);
}

int main() {
    int a[] = {1, 5, 9, 10, 15, 20};
    int b[] = {2, 3, 8, 13};
    int n = sizeof(a) / sizeof(a[0]);
    int m = sizeof(b) / sizeof(b[0]);
    mergeArrays(a, n, b, m);

    for (int i = 0; i < n; i++)
        printf("%d ", a[i]);
    printf("\n");
    for (int i = 0; i < m; i++)
        printf("%d ", b[i]);
    return 0;
}

// C program to merge two sorted arrays a[] and b[] 
// without extra space Using swap and sort
#include <stdio.h>
#include <stdlib.h>

int cmp(const void* a, const void* b) {
    return (*(int*)a - *(int*)b);
}

void mergeArrays(int* a, int* b, int n, int m) {
    int i = n - 1, j = 0;

    // Swap smaller elements from b[] with larger elements from a[]
    while (i >= 0 && j < m) {
        if (a[i] < b[j])
            i--;
        else {
            int temp = b[j];
            b[j] = a[i];
            a[i] = temp;
            i--;
            j++;
        }
    }

    // Sort both arrays
    qsort(a, n, sizeof(int), cmp);
    qsort(b, m, sizeof(int), cmp);
}

int main() {
    int a[] = {1, 5, 9, 10, 15, 20};
    int b[] = {2, 3, 8, 13};
    int n = sizeof(a) / sizeof(a[0]);
    int m = sizeof(b) / sizeof(b[0]);
    mergeArrays(a, b, n, m);

    for (int i = 0; i < n; i++)
        printf("%d ", a[i]);
    printf("\n");
    for (int i = 0; i < m; i++)
        printf("%d ", b[i]);
    return 0;
}

// Merge two sorted arrays a[] and b[] with O(1) extra space.
// using Gap method of Shell sort
#include <stdio.h>

void mergeArrays(int* a, int* b, int n, int m) {
    int gap = (n + m + 1) / 2;

    while (gap > 0) {
        int i = 0, j = gap;

        while (j < n + m) {
          
            // If both pointers are in the first array a[]
            if (j < n && a[i] > a[j]) {
                int temp = a[i];
                a[i] = a[j];
                a[j] = temp;
            } 
          
            // If first pointer is in a[] and 
            // the second pointer is in b[]
            else if (i < n && j >= n && a[i] > b[j - n]) {
                int temp = a[i];
                a[i] = b[j - n];
                b[j - n] = temp;
            } 
          
            // Both pointers are in the second array b
            else if (i >= n && b[i - n] > b[j - n]) {
                int temp = b[i - n];
                b[i - n] = b[j - n];
                b[j - n] = temp;
            }
            i++;
            j++;
        }

        // After operating for gap of 1 break the loop
        if (gap == 1) break;

        // Calculate the next gap
        gap = (gap + 1) / 2;
    }
}

int main() {
    int a[] = {1, 5, 9, 10, 15, 20};
    int b[] = {2, 3, 8, 13};
    int n = sizeof(a) / sizeof(a[0]);
    int m = sizeof(b) / sizeof(b[0]);
    mergeArrays(a, b, n, m);

    for (int i = 0; i < n; i++)
        printf("%d ", a[i]);
    printf("\n");
    for (int i = 0; i < m; i++)
        printf("%d ", b[i]);
    return 0;
}