java Iterativel preorder在二叉树中遍历

Posted 2021-05-07

# Preorder traversal: node -> node.left -> node.right
public void preorder(TreeNode root) {
    
    Stack<TreeNode> stack = new Stack<>();
    
    TreeNode current = root;
    
    while (stack.size > 0 || current != null) {
        if (current != null) {
            visit(current);
            stack.push(current);
            current = current.left;
        } else {
            current = stack.pop();
            current = current.right;
        }
    }
}

// second implementation
public void preorder2(TreeNode root) {
    Stack<TreeNode> stack = new Stack<>();
    
    TreeNode current;
    
    stack.push(root);
    
    while (stack.size() > 0) {
        current = stack.pop();
        visit(current);
        
        if (current.left != null) stack.push(current.left);
        if (current.right != null) stack.push(current.right);
    }
}