Program 16 - Expression Tree

	#include <stdio.h>
	#include <stdlib.h>
	struct node {
	char data;
	struct node *left, *right;
	};
	struct node *build_tree(char postfix[]) {
	struct node *stack[100], *nn;
	int i, top = -1;
	char ch;
	for(i = 0; postfix[i] != '\0'; i++) {
	ch = postfix[i];
	nn = (struct node *)malloc(sizeof(struct node));
	nn -> data = ch;
	nn -> left = nn -> right = NULL;
	if(isdigit(ch) || isalpha(ch)) {
	stack[++top] = nn;
	} else if(ch == '+' || ch == '-' || ch == '*' || ch == '/') {
	nn -> right = stack[top--];
	nn -> left = stack[top--];
	stack[++top] = nn;
	} else {
	printf("Invalid Expression\n");
	}
	}
	return stack[top--];
	}
	void inorder(struct node *root) {
	if(root != NULL) {
	inorder(root -> left);
	printf("%c", root -> data);
	inorder(root -> right);
	}
	}
	void preorder(struct node *root) {
	if(root != NULL){
	printf("%c", root -> data);
	preorder(root -> left);
	preorder(root -> right);
	}
	}
	void postorder(struct node *root) {
	if(root != NULL) {
	postorder(root -> left);
	postorder(root -> right);
	printf("%c", root -> data);
	}
	}
	int main() {
	struct node *root;
	char postfix[20];
	printf("Enter postfix expression: \n");
	gets(postfix);
	root = build_tree(postfix);
	printf("Inorder: ");
	inorder(root);
	printf("\nPreorder: ");
	preorder(root);
	//And now *drumroll*
	printf("\nPostorder: ");
	postorder(root);
	}