Flip Array Recursively | 易学教程

问题

There is this problem on LeetCode that I can not get to work in C/C++
The idea is to reverse an array in its place (using no other additional array) using recursion.
The link is : https://leetcode.com/explore/learn/card/recursion-i/250/principle-of-recursion/1440/
The solution is done in Java or Python.
I tried implementing the solution in C but I always get the original array, my code is as follows:

void reverseString(char* s, int sSize){
    if(!s)
        return;
    reverseString(s+1,sSize-1);
    s[sSize] = *s;
}

There is something I am not accounting for. Please let me know how would you solve it, and if possible why this is not working. Thanks.

回答1:

I'll take a stab at this.

The general idea for a recursive solution is for each call to get a pointer to the start of a string, and how many characters to look at, and this walks its way to the middle of the string.

void reverseString(char *start, int n)
{
    if (n <= 1) return;

    char tmp = start[0];
    start[0] = start[--n];   // the nth character is start[n-1]
    start[n] = tmp;

    reverseString(++start, --n);
}

On each recursive call, the starting string pointer is incremented by one, and the length decreased by two.

FIRST CALL:   v          v
              hello, world
SECOND CALL:   ^        ^

The common danger area is making sure it does the right thing with even and odd-length strings.

This method is a bit simpler with just two parameters, and - as some might say - a bit more elegant :-) even if the ++ and -- could be considered tricky (one increment and two decrements).

EDIT: This version is also tail recursive, which can lead to certain optimizations by internally turning it into a loop.

回答2:

Solution (thank you to the folks in the comments):

void reverse(char * str, int len)
{
    char tmp;

    if (len <= 1)
        return;

    tmp = str[0];
    len--;
    str[0] = str[len];
    str[len] = tmp;
    str++;
    reverse(str, len-1);
}

Call this function with your initial string and 0 as arguments:

char str[] = "Ding dong";

reverse(str, 0, strlen(a));

回答3:

void reverse_string(char *x, int start, int end)
{
    char ch;
    if (start >= end)
       return;

    ch = *(x+start);
    *(x+start) = *(x+end);
    *(x+end) = ch;

    //Function calling itself: Recursion
    reverse_string(x, ++start, --end);
}

In this function we are passing the string, the starting index and the ending index.... The recursion will continue till start>=end i.e. till the center of the string appears..... And everytime it will swap the 2 indexes i.e. from the start and from the end...

来源：https://stackoverflow.com/questions/59438783/flip-array-recursively

标签

recursion