Inorder, Preorder and Postorder Tree Traversals

#include <stdio.h>
#include <stdlib.h>
struct tree{
    int data;
    struct tree *left,*right;
}*root = NULL;
void insert(int val){
    struct tree *newNode = (struct tree*) malloc(sizeof(struct tree));
    newNode->data = val; newNode->right = newNode->left = NULL;
    if(!root) root = newNode;
    else{
        struct tree *help = root,*previous = NULL;
        while(help){//traversing to find insertion point
            previous = help;
            if(val < help->data) help = help ->left;
            else if(val > help->data) help = help->right;
            else{
                printf("\nDublicate values not allowed.\n");
                return;
            }
        }
        if(val > previous->data)   previous->right = newNode;
        else    previous->left = newNode;
    }
}
void inorder(struct tree* help){
    if(help){
        inorder(help->left);
        printf(" |%d| ", help->data);
        inorder(help->right);
    }
    if(!root)   printf("The tree is empty.\n");
}
void preorder(struct tree* help){
    if(help){
        printf(" |%d| ", help->data);
        preorder(help->left);
        preorder(help->right);
    }
    if(!root)   printf("The tree is empty.\n");
}
void postorder(struct tree* help){
    if(help){
        postorder(help->left);
        postorder(help->right);
        printf(" |%d| ", help->data);
    }
    if(!root)   printf("The tree is empty.\n");
}
int main(){
    int n, choice;
    while(1){
        printf("\n\n1.Insert 2.Inorder 3.Preorder 4.Postorder 5.Exit: ");
        scanf("%d", &choice);
        switch(choice){
            case 1: printf("\nInsert: ");
                    scanf("%d",&n);
                    insert(n);
                    break;
            case 2: printf("The tree by Inorder traversal is:-\n");
                    inorder(root);
                    break;
            case 3: printf("The tree by Prerder traversal is:-\n");
                    preorder(root);
                    break;
            case 4: printf("The tree by Postorder traversal is:-\n");
                    postorder(root);
                    break;
            case 5: return 0;
        }
    }
}