二叉树遍历

递归

前序：

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

中序和后序只需要改变第 9 行的位置即可；

迭代

前序遍历

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

中序遍历

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        Deque<TreeNode> stack = new ArrayDeque<>();
        TreeNode node = root;
        while (!stack.isEmpty() || node != null) {
            while (node != null) {
                stack.push(node);
                node = node.left;
            }
            node = stack.pop();
            res.add(node.val);
            node = node.right;
        }
        return res;
    }
}

后序遍历

前序遍历中左右改成中右左之后结果倒着输出

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        TreeNode node = root;
        Deque<TreeNode> stack = new ArrayDeque<>();
        stack.push(node);
        while (!stack.isEmpty()) {
            node = stack.pop();
            res.add(0, node.val);
            if (node.left != null) {
                stack.push(node.left);
            }
            if (node.right != null) {
                stack.push(node.right);
            }
        }
        return res;
    }
}