6.c

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

char*convert(char*s, intnRows) {
intlen=strlen(s);
char*ret= (char*)malloc(len+1);
inti, j;
j=0; /* index of return string */

/**
 * don't know why return s or modify input pointer will cause runtime error, 
 * so I have to copy a string and try not to modify s. I tried 5 times to
 * finally find out what happened, sad for my acceptance rate...
 */
if (nRows==1) {
while (j<len) {
ret[j] =s[j];
j++;
 }
ret[len] ='\0';
returnret;
 }

intt= (nRows-1) *2;
intshift, offset;

for (i=0; i<nRows; i++) {
offset=0;
shift= (nRows-i-1) *2;
while (j<len&&i+offset<len) {
ret[j] =s[i+offset];
j++;

if (shift==0) {
shift=t;
 }

offset+=shift;

if (shift!=t) {
shift=t-shift;
 }
 }
 }

ret[len] ='\0';

returnret;
}

intmain() {
charstr1[] ="PAYPALISHIRING";
charstr2[] ="A";
charstr3[] ="zvmwnuufnnxvloyvgmliuqandlyavfauaosnlnv";
printf("%s\n", convert(str1, 3));
printf("%s\n", convert(str1, 4));
printf("%s\n", convert(str1, 5));
printf("%s\n", convert(str2, 1));
printf("%s\n", convert(str3, 1));
return0;
}