144 Binary Tree Preorder Travesal

递归

public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> res = new ArrayList<Integer>();
        traversal(root, res);
        return res;
    }
    
    public void traversal(TreeNode root, ArrayList<Integer> res) {
        if (root == null) {
            return;
        }
        
        res.add(root.val);
        traversal(root.left, res);
        traversal(root.right, res);
    }
}

非递归

public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        ArrayList<Integer> preorder = new ArrayList<Integer>();
        
        if (root == null) {
            return preorder;
        }
        
        stack.push(root);
        
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            preorder.add(node.val);
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
        return preorder;
    }
}

divide and conquer

    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> res= new ArrayList<Integer>();
        if (root == null) {
            return res;
        }
        
        ArrayList<Integer> left = preorderTraversal(root.left);
        ArrayList<Integer> right = preorderTraversal(root.right);
        
        res.add(root.val);
        res.addAll(left);
        res.addAll(right);
        return res;
    }