144. Binary Tree Preorder Traversal

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DFS

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class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        // root -> left -> right

        List<Integer> list = new LinkedList<>();

        recur(list, root);

        return list;
    }

    private void recur(List<Integer> list, TreeNode node) {
        if (node == null) {
            return;
        }

        list.add(node.val);
        recur(list, node.left);
        recur(list, node.right);
    }
}

Iterate

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class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        // root -> left -> right

        List<Integer> resultList = new ArrayList<>();

        Stack<TreeNode> stack = new Stack<>();
        if (root != null) {
            stack.push(root);
        }

        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            resultList.add(node.val);

            if (node.right != null) {
                // 右节点先入栈，后出栈，保证在左节点之后遍历
                stack.push(node.right);
            }
            if (node.left != null) {
                // 左节点后入栈，先出栈，保证在右节点之前遍历
                stack.push(node.left);
            }
        }

        return resultList;
    }
}

Iterate

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class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        // root -> left -> right

        List<Integer> resultList = new ArrayList<>();

        Stack<TreeNode> stack = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                resultList.add(root.val);
                root = root.left;
            }

            // now: root is null
            TreeNode node = stack.pop(); // 回溯
            if (node.right != null) {
                root = node.right;
            }
        }

        return resultList;
    }
}

References

144. Binary Tree Preorder Traversal