如何检查正确的 PN 表达式？
#include <stdio.h>
#include <stdlib.h>
struct sNode {
char data;
struct sNode *next;
};
void push(struct sNode** top_ref, int new_data) {
printf("Insert to stack %d\n", new_data);
struct sNode* new_node = (struct sNode*) malloc(sizeof(struct sNode));
if(new_node == NULL) {
printf("Stack overflow \n");
getchar();
exit(0);
}
new_node->data = new_data;
new_node->next = (*top_ref);
(*top_ref) = new_node;
}
int pop(struct sNode** top_ref) {
char res;
struct sNode *top;
if(*top_ref == NULL) {
printf("Stack overflow \n");
getchar();
exit(0);
} else {
top = *top_ref;
res = top->data;
*top_ref = top->next;
free(top);
printf("Remove from stack %d\n",res);
return res;
}
}
void evaluate_rpm() {
char c;
struct sNode *stack = NULL;
char number[10] = "";
number[9] = 0;
int i = 0, val1, val2;
do {
c = (char) getchar();
if (c >= '0' && c <= '9') {
number[i++] = c;
} else if (c == '*') {
val1 = pop(&stack);
val2 = pop(&stack);
push(&stack, val1 * val2);
} else if (c == '/') {
val1 = pop(&stack);
val2 = pop(&stack);
if (val1 == 0) {
fprintf(stderr, "[!] Illegal divide by zero. Exiting...\n");
exit(0);
}
push(&stack, val2 / val1);
} else if (c == '-') {
val1 = pop(&stack);
val2 = pop(&stack);
push(&stack, val2 - val1);
} else if (c == '+') {
val1 = pop(&stack);
val2 = pop(&stack);
push(&stack, val1 + val2);
} else if (c == ' ' && i != 0) {
val1 = atoi(number);
push(&stack, val1);
int j;
for (j = 0; j <= i; j++) {
number[j] = 0;
}
i = 0;
} else if (c == '\n') {
val1 = pop(&stack);
printf("=========================\n");
printf("result of evaluation %d\n", val1);
printf("=========================\n");
} else if (c != ' ' && c != EOF) {
printf("%d\n", (int)c);
fprintf(stderr, "[!] Syntax error in input. Exiting...\n");
exit(1);
}
} while (c != EOF);
}
int main() {
printf("Enter an expression.\n");
evaluate_rpm();
return 0;
}