Print root to leaf paths without using recursion - GeeksforGeeks


Print root to leaf paths without using recursion - GeeksforGeeks
Given a binary tree, print all its root to leaf paths without using recursion. For example, consider the following Binary Tree.

        6
     /    \
    3      5
  /   \     \
 2     5     4
     /   \
    7     4

There are 4 leaves, hence 4 root to leaf paths -
  6->3->2              
  6->3->5->7
  6->3->5->4
  6->5>4
how to extend the traversal to print root to leaf paths? The idea is to maintain a map to store parent pointers of binary tree nodes. Now whenever we encounter a leaf node while doing iterative preorder traversal, we can easily print root to leaf path using parent pointer
/* Function to print root to leaf path for a leaf
   using parent nodes stored in map */
void printTopToBottomPath(Node* curr,
                         map<Node*, Node*> parent)
{
    stack<Node*> stk;
 
    // start from leaf node and keep on pushing
    // nodes into stack till root node is reached
    while (curr)
    {
        stk.push(curr);
        curr = parent[curr];
    }
 
    // Start popping nodes from stack and print them
    while (!stk.empty())
    {
        curr = stk.top();
        stk.pop();
        cout << curr->data << " ";
    }
    cout << endl;
}
 
/* An iterative function to do preorder traversal
   of binary tree  and print root to leaf path
   without using recursion */
void printRootToLeaf(Node* root)
{
    // Corner Case
    if (root == NULL)
        return;
 
    // Create an empty stack and push root to it
    stack<Node*> nodeStack;
    nodeStack.push(root);
 
    // Create a map to store parent pointers of binary
    // tree nodes
    map<Node*, Node*> parent;
 
    // parent of root is NULL
    parent[root] = NULL;
 
    /* Pop all items one by one. Do following for
       every popped item
        a) push its right child and set its parent
           pointer
        b) push its left child and set its parent
           pointer
       Note that right child is pushed first so that
       left is processed first */
    while (!nodeStack.empty())
    {
        // Pop the top item from stack
        Node* current = nodeStack.top();
        nodeStack.pop();
 
        // If leaf node encountered, print Top To
        // Bottom path
        if (!(current->left) && !(current->right))
            printTopToBottomPath(current, parent);
 
        // Push right & left children of the popped node
        // to stack. Also set their parent pointer in
        // the map
        if (current->right)
        {
            parent[current->right] = current;
            nodeStack.push(current->right);
        }
        if (current->left)
        {
            parent[current->left] = current;
            nodeStack.push(current->left);
        }
    }
}

https://www.quora.com/How-do-I-print-all-root-to-leaf-paths-in-binary-tree-iteratively

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