Immediate Decodability UVA-644(qsort排序 + 模拟)

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
char code[1000][100];
bool solve(int p)
{
    for(int i = 1; i < p; i++)
    {
        int len = min(strlen(code[i-1]), strlen(code[i]));
        int j;
        for(j = 0; j < len; j++)
        {
            if(code[i-1][j] != code[i][j])
                break;
        }
        if(j == len)
            return false;
    }
    return true;
}
int cmp(void const *va, void const *vb)
{
    char *a = (char*)va;
    char *b = (char*)vb;
    return strcmp(a, b);
}
int main()
{
    int d = 0;
    while(scanf("%s", code[0]) != EOF)
    {
        int p = 1;
        while(scanf("%s", code[p]) && code[p][0] != '9') { p++;}
        qsort(code, p, sizeof(code[0]), cmp);       /*这里sizeof内的要注意*/
        if(solve(p))
            printf("Set %d is immediately decodable\n", ++d);
        else
            printf("Set %d is not immediately decodable\n", ++d);
    }
}

qsort介绍：https://www.runoob.com/cprogramming/c-function-qsort.html

思考：为什么要先排序再查找，不排序为何样例不通过？