UVa 442 - Matrix Chain Multiplication

/*****************************************
  Filename:p442.c
  Author:Willy Chen(willychen.org)
  Date:2018.04.19
*****************************************/
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

struct m{
    char name;
    int row;
    int col;
};
typedef struct m Matrix;

struct s{
    Matrix matrix;
    struct s* downPtr;
};
typedef struct s Stack;

// Check if the stack is empty
int isEmpty(Stack **stack){
    if(stack == NULL)
        return 1;
    else
        return 0;
}

void push(Stack **stack,Matrix matrix){
    //printf("Insert:%c %d %d\n",name,row,col);
    Stack * new_stack = (Stack*) malloc(sizeof(Stack));
    if(new_stack!=NULL){
        new_stack->matrix = matrix;
        new_stack->downPtr = (*stack);
        (*stack) = new_stack;
    }
}

// Check if stack is empty before pop function
Matrix pop(Stack **stack){
    Stack *top = *stack;
    Matrix matrix = top->matrix;
    *stack = (*stack)-> downPtr;
    free(top);
    return matrix;
};

// Show the stack
void show(Stack **stack){
    Stack *now = *stack;
    while(now!=NULL){
        Matrix matrix = now->matrix;
        printf("%c %d %d -> ",matrix.name,matrix.row,matrix.col);
        now = now->downPtr;
    }
    printf("NULL\n");
}

int main(){
    int num;
    scanf("%d\n",&num);
    //printf("num:%d\n",num);
    Matrix matrix[30];

    for(int i=0;i<num;i++){
        char name;
        int row,col;
        scanf("%c%d%d\n",&name,&row,&col);
        matrix[i].name = name;
        matrix[i].row = row;
        matrix[i].col = col;
    }

    // Show all matrices
    /*
    for(int i=0;i<num;i++){
        printf("%c %d %d\n",matrix[i].name,matrix[i].row,matrix[i].col);
    }
    */

    char command[10000]={0};
    while(scanf("%s",command)!=EOF){
        int ans=0;
        int flag=1;
        //stack of matrix
        Stack* stack = NULL;

        int len = strlen(command);
        for(int i=0;i<len;i++){
            if(command[i]=='(')
                continue;
            else if(command[i]==')'){
                // Pop 2 matrices and calculate
                Matrix m1,m2;
                if(isEmpty(&stack)){
                    flag=0;
                    break;
                }else{
                    m2 = pop(&stack);
                }
                if(isEmpty(&stack)){
                    flag=0;
                    break;
                }else{
                    m1 = pop(&stack);
                }
                if(m1.col!=m2.row){
                    flag=0;
                    break;
                }
                ans += m1.row*m1.col*m2.col;
                m1.col = m2.col;
                // Push the result on top of stack
                push(&stack,m1);
            }else{
                // Find Matrix
                int j=0;
                for(j=0;j<num;j++)
                    if(command[i]==matrix[j].name)
                        break;
                // Push Matrix on the top of stack
                push(&stack,matrix[j]);
            }
            //show(&stack);
        }
        if(flag)
            printf("%d\n",ans);
        else
            printf("error\n");

        // Reset
        memset(command,0,sizeof(command)/sizeof(char));
    }

    return 0;
}