Infix to Postfix

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct node {
    char data;
    struct node * next;
};

void push (struct node **top, char c) {
    struct node *temp = (struct node*)malloc(sizeof(struct node));
    temp -> next = NULL;
    temp->data = c;
    temp -> next = *top;
    *top = temp;
}

int pop (struct node **top) {
    struct node *p = *top;
    if(p!=NULL) {
        int popped = p->data;
        *top = p->next;
        return popped;
    }
    else {
        printf("Empty Stack");
        return -999;
    }
}

int peak(struct node **top) {
    return ((*top)->data);
}

int main() {
    struct node * top = NULL;
    int x=0;
    printf("Enter the FPE Infix expression: ");
    char exp[100];
    char out[100];
    gets(exp);
    for(int i=0; i< strlen(exp); i++) {
        if(exp[i]=='(')
        push(&top, '(');
        else if ((exp[i]>=65 && exp[i]<=90)||(exp[i]>=48 && exp[i]<=57)) {
            out[x++] = exp[i];
        }
        else if(exp[i]==')') {
            while(peak(&top)!='(') {
                out[x++] = pop(&top);
            }
            char temp = pop(&top);
        }
        else {
            if(exp[i]=='+'||exp[i]=='-') {
                if(peak(&top)=='*'||peak(&top)=='/'||peak(&top)=='^') {
                    out[x++] = pop(&top);
                    push(&top, exp[i]);
                }
                else {
                    push(&top, exp[i]);
                }
            }
            else if(exp[i]=='/'||exp[i]=='*') {
                if(peak(&top)=='^') {
                    out[x++] = pop(&top);
                    push(&top, exp[i]);
                }
                else {
                    push(&top, exp[i]);
                }
            }
            else {
                push(&top, exp[i]);
            }
        }
        }

        printf("Output: %s", out);

    }