hihocoder 1197 Give My Text Back

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <string.h>
#include <limits.h>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <algorithm>

using namespace std;


char buf[10000];

bool iszimu(char c) {
    return ((c>='a' && c<='z') || (c>='A' && c<='Z'));
}
char turn_capital(char c) {
    if (c>='a' && c<='z') {
        c = c + 'A' - 'a';
    }
    return c;
}
char turn_small(char c) {
    if (c>='A' && c<='Z') c = c+'a'-'A';
    return c;
}
int main()
{
    while(scanf("%[^\n]", buf)>0){
        //printf("%s\n", buf);
        char str[10000] = {0};
        int j = 0;
        for (int i=0;buf[i]!=0;++i) {
            if (iszimu(buf[i])) {
                if (j==0 || str[j-2] == '.') {
                    str[j] = turn_capital(buf[i]);
                } else {
                    str[j] = turn_small(buf[i]);
                }
            } else if (buf[i] == ' ') {
                if (j==0) continue;
                if (str[j-1] != ' ') str[j] = buf[i];
                else j--;
            } else if (buf[i] == ',' || buf[i] == '.') {
                if (str[j-1] == ' ') {
                    str[j-1] = buf[i];
                    str[j] = ' ';
                } else {
                    str[j] = buf[i];
                    str[j+1] = ' ';
                    j++;
                }
            }
            j++;
        }
        printf("%s\n", str);
        getchar();
    }
}