Codeforces Round #313 (Div. 2) D. Equivalent Strings 解题心得

Today on a lecture about strings Gerald learned a new definition of string equivalency. Two strings a and b of equal length are called equivalent in one of the two cases:

1. They are equal.
2. If we split string a into two halves of the same size a₁ and a₂, and string b into two halves of the same sizeb₁ and b₂, then one of the following is correct:
   1. a₁ is equivalent to b₁, and a₂ is equivalent to b₂
   2. a₁ is equivalent to b₂, and a₂ is equivalent to b₁

As a home task, the teacher gave two strings to his students and asked to determine if they are equivalent.

Gerald has already completed this home task. Now it‘s your turn!

The first two lines of the input contain two strings given by the teacher. Each of them has the length from 1 to200 000 and consists of lowercase English letters. The strings have the same length.

Print "YES" (without the quotes), if these two strings are equivalent, and "NO" (without the quotes) otherwise.

In the first sample you should split the first string into strings "aa" and "ba", the second one — into strings "ab" and "aa". "aa" is equivalent to "aa"; "ab" is equivalent to "ba" as "ab" = "a" + "b", "ba" = "b" + "a".

In the second sample the first string can be splitted into strings "aa" and "bb", that are equivalent only to themselves. That‘s why string "aabb" is equivalent only to itself and to string "bbaa".

我的代码

#include<cstdio>
#include<cstring>
int const Max = 250000;
char a[Max], b[Max];

bool isequal(char *a, char *b, int length)
{
    if (!strncmp(a, b, length))
        return true;
    if (length % 2)
        return false; 
    int mid = length / 2;
    if (isequal(a, b + mid, mid) && isequal(a + mid, b, mid))
        return true;
    if (isequal(a, b, mid) && isequal(a + mid, b + mid, mid))
        return true;
    return false;

}

int main()
{
    scanf("%s%s", a, b);
    printf("%s\n", isequal(a, b, strlen(a)) ? "YES" : "NO");

}

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