51nod1006的ac代码，我下面的代码有啥问题？

Question

`#include<stdio.h>

include<string.h>

define N 1001

char str1[N];
char str2[N];
char str[N];
int dpN;
int max(int a, int b) {

return a > b ? a : b;

}

int main() {

int i, j, len1, len2;
scanf("%s %s", str1+1, str2+1);
memset(dp, 0, sizeof(dp));
len1 = strlen(str1+1);
len2 = strlen(str2+1);

for(i = 1; i <= len1; i++) {
    for (j = 1; j <= len2; j++) {
        if (str1[i] == str2[j])
            dp[i][j] = dp[i-1][j-1] + 1;
        else dp[i][j] = max(dp[i-1][j], dp[i][j-1]);
    }
}

int k = len1, m = len2, num = 0;
while (k&&m) {
    if (str1[k] == str2[m]) {
        str[num] = str[k];
        num++;
        k--;
        m--;
    } else if (dp[k][m] == dp[k-1][m]) {
        k--;
    } else if (dp[k][m] == dp[k][m-1]) {
        m--;
    }
}
for(int q = num-1; q >= 0; q--) {
    printf("%c", str[q]);
}
printf("\n");
return 0;

}
`