C++ Depth-First Search Algorithm for Permutations

This code utilizes a depth-first search (DFS) algorithm to generate all possible permutations of the numbers 1, 2, and 3. It employs a boolean array 'mark' to keep track of which numbers have been used in the current permutation and an integer array 'A' to store the current permutation.

The 'dfs' function is a recursive function that takes an integer 'k' as input. If 'k' equals 4, it indicates that all 3 numbers have been placed in the permutation, so the function prints the permutation and returns. Otherwise, it iterates through the numbers 1, 2, and 3 and checks if each number has been used ('mark[i] == 0'). If a number hasn't been used, it marks it as used ('mark[i] = 1'), places it in the current permutation ('A[k] = i'), recursively calls 'dfs' with 'k+1', and then marks the number as unused again ('mark[i] = 0'). This process generates all possible permutations of the numbers 1, 2, and 3.

In the 'main' function, 'dfs(1)' is invoked to initiate the generation of permutations, and the program returns 0 to signify successful execution.