No. 19 - Left Rotation of String

No. 19 - Left Rotation of String

Problem: Left rotation of a string is to move some leading characters to its tail. Please implement a function to rotate a string. 

For example, if the input string is “abcdefg” and a number 2, the rotated result is “cdefgab”.

Analysis: It looks difficult to get rules of left rotation on a string. Fortunately, the 7^th problem in this series “Reverse Words in a Sentence” can give us some hints.

If we input a sentence with two words “hello world” for the problem “Reverse Words in a Sentence”, the reversed result should be “world hello”. It is noticeable that the result “world hello” can be viewed as a rotated result of “hello world”. It becomes “world hello” when we move some leading characters of string “hello world” to its ending. Therefore, this problem is quite similar to problem “Reverse Words in a Sentence”.

Let us take a string “abcdefg” as an example. We divide it into two parts: the first part contains the two leading characters “ab”, and the second part contains all other characters “cdefg”. We firstly reverse these two parts separately, and the whole string becomes “bagfedc”. It becomes “cdefgab” if we reverse the whole string, which is the expected result of left rotation with 2.

According to the analysis above, we can see that left rotation of a string can be implemented calling a Reverse function three times to reverse a segment or whole string. The sample code is shown below:

char* LeftRotateString(char* pStr, int n)

{

    if(pStr != NULL)

    {

        int nLength = static_cast<int>(strlen(pStr));

        if(nLength > 0 && n > 0 && n < nLength)

        {

            char* pFirstStart = pStr;

            char* pFirstEnd = pStr + n - 1;

            char* pSecondStart = pStr + n;

            char* pSecondEnd = pStr + nLength - 1;

            // Reverse the n leading characters

            Reverse(pFirstStart, pFirstEnd);

            // Reverse other characters

            Reverse(pSecondStart, pSecondEnd);

            // Reverse the whole string

            Reverse(pFirstStart, pSecondEnd);

        }

    }

    return pStr;

}

The function Reverse is shown in “Reverse Words in a Sentence”, so we are not going to repeat it here.

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