Rotate an Array with the Reversal Algorithm

PROBLEM UNDERSTANDING

Input and expected output

No input required

3 4 5 6 7 1 2

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>

void reverse(int values[], int left, int right)
{
    while (left < right) {
        int temporary = values[left];
        values[left++] = values[right];
        values[right--] = temporary;
    }
}

int main(void)
{
    int values[] = {1, 2, 3, 4, 5, 6, 7};
    int length = 7, positions = 2;
    reverse(values, 0, positions - 1);
    reverse(values, positions, length - 1);
    reverse(values, 0, length - 1);
    for (int index = 0; index < length; index++) printf("%d ", values[index]);
    putchar('\n');
    return 0;
}

PROGRAM EXPLANATION

Algorithm and explanation

1. Start with {1, 2, 3, 4, 5, 6, 7}; length is 7 and positions is 2.
2. Reverse indices 0 through 1 to get {2, 1, 3, 4, 5, 6, 7}.
3. Reverse indices 2 through 6 to get {2, 1, 7, 6, 5, 4, 3}.
4. Reverse the whole array to get {3, 4, 5, 6, 7, 1, 2}, then print it.

Write the array as two segments A and B, where A contains the first k elements. Reversing A, then B, then the complete array transforms reverse(A) + reverse(B) into B + A. Each reversal swaps the two outer elements and moves the endpoints inward. The total work is O(n), and the temporary variable needs O(1) auxiliary space. The supplied example uses a fixed array and does not read input. Its indices are valid for 0 <= positions <= length. For a general API with length > 0, normalize a non-negative rotation count using positions %= length; handle an empty array before taking the remainder.

EFFICIENCY

Time and space complexity

DEBUGGING CHECKLIST

Common mistakes

Try it yourself