Question

1. Write a program in C to show how to traverse a tree or visit each node in the tree exactly once? Use the following figure to implement the three methods, LVR (inorder), LRV (postorder), VLR (preorder). 17 1819 14 12 8 Figure 5.15: Binary tree with arithmetic expression

Answer 1

Assuming TreeNode has following structure:
struct Node {
    char value;    // value stored in node
    Node * left;   // left child
    Node * right;  // right child
};

===============================
Below methods should be called
with input paramter root of tree.
===============================
void inorder(TreeNode * t) {
  if(t != NULL) {
    inorder(t->left);
    printf("%c ", t->value);
    inorder(t->right);
  }
}

/* should visit the nodes in preorder fashion */
void preorder(TreeNode * t) {
  if(t != NULL) {
    printf("%c ", t->value);
    preorder(t->left);
    preorder(t->right);
  }
}

void postorder(Node * t) {
  if(t != NULL) {
    preorder(t->left);
    preorder(t->right);
    printf("%c ", t->value);
  }
}

============
Working code:

#include <iostream>

using namespace std;

struct TreeNode {
    char value;    // value stored in node
    TreeNode * left;   // left child
    TreeNode * right;  // right child
};

/*===============================
Below methods should be called
with input paramter root of tree.
===============================*/
void inorder(TreeNode * t) {
  if(t != NULL) {
    inorder(t->left);
    printf("%c ", t->value);
    inorder(t->right);
  }
}

/* should visit the nodes in preorder fashion */
void preorder(TreeNode * t) {
  if(t != NULL) {
    printf("%c ", t->value);
    preorder(t->left);
    preorder(t->right);
  }
}

void postorder(TreeNode * t) {
  if(t != NULL) {
    preorder(t->left);
    preorder(t->right);
    printf("%c ", t->value);
  }
}

int main() {
        TreeNode *root = new TreeNode;
        root->value = '+';
        root->right = new TreeNode;
        root->right->value = 'E';

        TreeNode* x = new TreeNode;
        x->value = '*';
        root->left = x;
        x->right = new TreeNode;
        x->right->value = 'D';

        TreeNode* y = new TreeNode;
        y->value = '*';
        x->left = y;
        x = y;
        x->right = new TreeNode;
        x->right->value = 'C';

        y = new TreeNode;
        y->value = '/';
        x->left = y;
        x = y;
        x->right = new TreeNode;
        x->right->value = 'B';

        y = new TreeNode;
        y->value = 'A';
        x->left = y;

        printf("\nInorder: ");
        inorder(root);
        printf("\nPreorder: ");
        preorder(root);
        printf("\nPostorder: ");
        postorder(root);
}