include stdio include stdlib include malloc include assert typedef str

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <assert.h>
typedef struct stack *Stack;
typedef struct node *Node;
Stack createStack();
int isEmpty(Stack stack);
int pushItem(Stack stack, char item);
char popItem(Stack stack);
int priority(char);
char *processExpression(char *expression);
struct node {
	char value;
	Node next;
};
struct stack {
	Node head;
};

Stack createStack(){
	Stack stack = malloc(sizeof(Stack));
	if (stack == NULL)
		return NULL;
	stack->head = NULL;
	return stack;
}
int isEmpty(Stack stack){
	assert(stack!=NULL);
	return stack->head == NULL;
}
int pushItem(Stack stack, char item){
	assert(stack != NULL);
	Node node = malloc(sizeof(Node));
	if (node == NULL)
		return 0;
	node->value=item;
	node->next=stack->head;
	stack->head=node;
	return 1;
}
char popItem(Stack stack){
	assert(stack!=NULL);
	if (stack->head ==NULL)
		return '\n';
	Node node = stack->head->next;
	char item = stack->head->value;
	free(stack->head);
	stack->head=node;
	return item;
}
int priority(char item){
	switch (item){
		case '*':
		case '/':
			return 3;
		case '-':
		case '+':
			return 2;
		case '(':
			return 1;
	}
}
char *processExpression(char *expression){
	Stack stack = createStack();
	char c;
	char *output = calloc(sizeof(char),80);
	int k,point;
	k = point=0;
	c = expression[k];
	while (c!='\0'){
		if (c==')'){
			while (stack->head->value!='(')
				output[point++]=popItem(stack);
			popItem(stack);
		}
		if (c>='a' && c<='z')
			output[point++]=c;
		if (c=='(')
			pushItem(stack,'(');
		if (c=='+'||c=='-'||c=='*'||c=='/'){
			if (isEmpty(stack))
				pushItem(stack,c);
			else{
				while(!isEmpty(stack)&&(priority(stack->head->value)>=priority(c)))
					output[point++]=popItem(stack);
				pushItem(stack,c);
			}
		}
		k++;
		c = expression[k];
	}
	while (stack->head!=NULL)
		output[point++]=popItem(stack);
	output[point]='\0';
	free(stack);
	return output;
}
int main(){
	printf("a+b=%s\n",processExpression("a+b\0"));
	printf("(a+b)*(c-d)-e=%s\n", processExpression("(a+b)*(c-d)-e\0"));
	printf("a+b*c/(d-e)*f=%s\n", processExpression("a+b*c/(d-e)*f\0"));
	return 0;
}