[Daily] Algorithm-rotation string-three-step flip method

Title Description

Given a string, it is required to move several characters in front of the string to the end of the string, such as the string "ABCdef" before the 2 characters ' a ' and ' B ' moved to the end of the string, so that the original string into the string "Cdefab". Write a function that requires an O (n) time complexity for a string operation of length n and an O (1) space complexity.

2:3-Step inversion method for analysis and solution

For this problem, think for a different angle.

Dividing a string into X and y two parts, defining a reversal operation on each part of the string, such as x^t, which reverses all the characters of x (for example, x= "abc", then x^t= "CBA"), then the following conclusion is obtained: (x^ty^t) ^t=yx, obviously solves the problem of string inversion.

For example, String abcdef, to let def flip to the head of ABC, just follow these 3 steps:

1. First of all, the original string is divided into two parts, namely X:abc,y:def;
2. The X is reversed, x->x^t,:abc->cba; Y is reversed, y->y^t, that is: def->fed.
3. Reverse the results of the above steps to get the string x^ty^t, that is, the inverse of the string cbafed two parts (CBA and fed) to give reversal, cbafed get DEFABC, formally expressed as (x^ty^t) ^t=yx, which realizes the entire reversal.

C language version:

#include <stdio.h> #include <string.h>void reversestring (char* s,int from,int to) {when    (from < to) c1/>{           char t = s[from];        s[from++] = s[to];        s[to--] = t;    }   } void Leftrotatestring (char* s,int n,int m) {    m%= n;               To move the left more than n bits, then the%n is equivalent to    reversestring (s, 0, m-1);//Invert [0..m-1], apply to the example above, is x->x^t, that    is ABC->CBA ReverseString (S, M, n-1); Invert [M.. N-1], such as y->y^t, i.e. def->fed    reversestring (s, 0, n-1);//Invert [0..n-1], i.e. the entire reversal, (x^ty^t) ^t=yx, CBAFED->DEFABC 。 }int Main () {        char a[]= "Hello World";        Leftrotatestring (A,strlen (a), 2);        printf ("%s\n", a);}

Go language version:

Package Mainimport (        "FMT") func reversestring (s []byte,from int,to int) {        for{                T:=s[from]                s[from]=s[ To]                s[to]=t                from++                to--                if from>to{                        break                }        }}func leftrotatestring (s []byte,n int , M int) {        m%=n        reversestring (s,0,m-1)        reversestring (s,m,n-1)        reversestring (s,0,n-1)}func main () {        a:=[]byte ("Hello World")        leftrotatestring (A,len (a), 2)        FMT. Println (String (a))}

PHP Version:

<?phpfunction reversestring (& $str, $from, $to) {while        ($from < $to) {                $t = $str [$from];                $str [$from ++]= $str [$to];                $str [$to--]= $t;        }   } Function leftrotatestring (& $str, $n, $m) {        $m%= $n;//If the length of the first few characters is greater than the string length        reversestring ($str, 0, $m-1);        ReverseString ($str, $m, $n-1);        ReverseString ($str, 0, $n-1);} $a = "Hello World"; Leftrotatestring ($a, strlen ($a), 2); Echo $a. " \ n ";

　　

　　

　　

[Daily] Algorithm-rotation string-three-step flip method