144.二叉树的前序遍历

递归

public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        traversal(root, list);
        return list;
    }

    public void traversal(TreeNode t, List<Integer> list) {
        if (t == null) {
            return;
        }
        list.add(t.val);
        traversal(t.left, list);
        traversal(t.right, list);
    }

迭代（不断走向左子树）

 public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode t = root;
        while (!stack.isEmpty() || t != null) {
            if (t != null) {
                list.add(t.val);
                stack.push(t);
                t = t.left;
            } else {
                // t已经被遍历过，再拿出来的时候需要遍历t的右子树
                t = stack.pop();
                t = t.right;
            }
        }
        return list;
    }

迭代（先放右子树，再放左子树）

public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        if(root==null)return list;
        TreeNode t = root;
        stack.push(t);
        while (!stack.isEmpty()) {
            t = stack.pop();
            list.add(t.val);
            if (t.right != null) {
                stack.push(t.right);
            }
            if (t.left != null) {
                stack.push(t.left);
            }
        }
        return list;
    }