请问为什么第5行换成string就wa又re，用char[]就ac?

#include<iostream>
#include<cmath>
using namespace std;
int cnt = 0;
char str[90000];
void print(int x) {
    int a = 0,b = 0;
    for(int i = 0;i < cnt;i++) {
        if(str[i] == 'W')a++;
        if(str[i] == 'L')b++;
        if((a>=x||b>=x)&&abs(a-b)>=2) {
            cout<<a<<":"<<b<<endl;
            a = 0;b = 0;
        }
    }
    cout<<a<<":"<<b<<endl;
}
int main() {
    char ch;
    while(cin>>ch&&ch!='E') {
        if(ch=='W'||ch=='L')str[cnt++] = ch;
    }
    print(11);
    cout<<endl;
    print(21);
    return 0;
}