Binary Search Tree Insertion & Deletion

#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){
            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 eliminate(int value){
    struct tree *help = root, *previous = help;
    while (help){ //start from root, traverse until help becomes null or node's value matches
        if(help->data==value) break;
        previous  = help;
        if (value > help-> data) help = help->right;
        else help = help->left;
    }
    if(!help) printf("Element not found!\n");
    else{
        struct tree *target = help;
        if (!target->left && !target->right) { //if node is a leaf node
            if(target == root)
                root = NULL;
            else if(previous->left == target) previous->left = NULL;
            else previous->right = NULL;
        }
        else if (!target->left || !target->right) { //if node has one child
            if(target == root){
                if(target->left)    root = root->left;
                else root = root->right;
            }
            if(previous->left == target){
                if(target->left)    previous->left = target->left;
                else previous->left = target->right;
            }
            else{
                if(target->left)    previous->right = target->left;
                else previous->right = target->right;
            }
        }
        else { //if node has 2 children
            struct tree* t = target; //t will point to leftmost leaf node of target's right subtree
            t = t->right;
            while(t->left){
               previous = t;
                t = t->left;
            }
            if(t->right){
                previous = t;
                t = t->right;
            }
            target->data = t->data;
            if(previous->right) previous->right = NULL;
            else previous->left = NULL;
            free(t);
        }
        free(target);
    }
}
void display(struct tree* help){
    if(help){
        display(help->left);
        printf(" |%d| ", help->data);
        display(help->right);
    }
    if(!root)   printf("The tree is empty.\n");
}
int main(){
    int n, choice;
    while(1){
        display(root);
        printf("\n\n1. Insert 2. Delete 3. Exit.\nInput: ");
        scanf("%d", &choice);
        switch(choice){
            case 1: printf("\nInsert: ");
                    scanf("%d",&n);
                    insert(n);
                    break;
            case 2: if(root){
                        printf("\nDelete: ");
                        scanf("%d",&n);
                        eliminate(n);
                    }
                    break;
            case 3: return 0;
        }
    }
}