反转移动到前方变换

原文:https://www . geeksforgeeks . org/reving-move-front-transform/

先决条件: 移动到前方数据变换算法

MTF 变换逆运算背后的主要思想:

1.计算 MTF 变换的逆就是撤销 MTF 变换，恢复原始字符串。我们有“input _ arr”和“n”，前者是 MTF 变换，“input _ arr”中的元素数量。

2.我们的任务是维护一个有序的字符列表(在我们的例子中是 a 到 z)，并从“input _ arr”中一次读入一个“ith”元素。

3.然后，以该元素为索引 j ，在列表中打印“jth”字符。

Illustration for "[15 1 14 1 14 1]"
List initially contains English alphabets in order.
We move characters at indexes depicted by input
to front of the list one by one.
input arr chars output str list
15 p abcdefghijklmnopqrstuvwxyz
1 pa pabcdefghijklmnoqrstuvwxyz
14 pan apbcdefghijklmnoqrstuvwxyz
1 pana napbcdefghijklmoqrstuvwxyz
14 panam anpbcdefghijklmoqrstuvwxyz
1 panama manpbcdefghijkloqrstuvwxyz


示例:

Input : arr[] = {15, 1, 14, 1, 14, 1}
Output : panama
Input : arr[] = {6, 5, 0, 10, 18, 8, 15, 18,
6, 6, 0, 6, 6};
Output : geeksforgeeks


下面是上面解释的 idea 代码:

// C program to find Inverse of Move to Front
// Transform of a given string
#include
#include
#include
// Takes index of printed character as argument
// to bring that character to the front of the list
void moveToFront(int index, char *list)
{
    char record[27];
    strcpy(record, list);
    // Characters pushed one position right
    // in the list up until index
    strncpy(list+1, record, index);
    // Character at index stored at 0th position
    list[0] = record[index];
}
// Move to Front Decoding
void mtfDecode(int arr[], int n)
{
    // Maintains an ordered list of legal symbols
    char list[] = "abcdefghijklmnopqrstuvwxyz";
    int i;
    printf("\nInverse of Move to Front Transform: ");
    for (i = 0; i     {
        // Printing characters of Inverse MTF as output
        printf("%c", list[arr[i]]);
        // Moves the printed character to the front 
        // of the list
        moveToFront(arr[i], list);
    }
}
// Driver program to test functions above
int main()
{
    // MTF transform and number of elements in it.
    int arr[] = {15, 1, 14, 1, 14, 1};
    int n = sizeof(arr)/sizeof(arr[0]);
    // Computes Inverse of Move to Front transform
    // of given text
    mtfDecode(arr, n);
    return 0;
}


输出:

Inverse of Move to Front Transform: panama


时间复杂度: O(n^2)

练习:在一个程序中一起实现 MTF 编码和解码，检查原始消息是否恢复。

来源:
http://www . cs . Princeton . edu/courses/archive/fall 07/cos226/assignments/burrows . html