求高手帮忙检查下为什么这个程序运行会发生错误

#include <iostream>
using namespace std;
const int _maxbill= 10;
const int _maxmoney =5000;
int a[_maxbill][_maxmoney];
int x[_maxbill];
int exchange(int num, int money){
    int min = 65535, number;
    if(num == 1)    return a[num-1][money]=money;
    else {
        number = money/x[num];
        for(int i = 1; i <= number; i++){
            int remain = money - x[i]*i;
            if(a[i-1][remain]== -1)    return exchange(i-1, remain);
            else min = (min > a[i][remain])? a[i][remain] : min;
        }
        return a[num][money] = min;
    }
}

int main() {
    int n, money;
    for(int i = 0; i<_maxbill; i++ )
        for(int j = 0; j<_maxmoney; i++)
            a[i][j]= -1;
    for(int i = 0; i<_maxbill; i++ )
        x[i] = -1;
    cout<<"请输入钱币的种类数："<<endl;
    cin>>n;