二叉树的遍历方法

前序、中序、后序遍历

前序遍历

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

                stack.push(node);
                stack.push(null);
            } else {
                node = stack.pop();
                res.add(node.val);
            }
        }
        return res;
    }
}

这是前序遍历的统一迭代法

如果是普通的方法

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

后序遍历

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

层序遍历

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        if(root == null) return new ArrayList<>();
        List<List<Integer>> result = new ArrayList<>();
        Deque<TreeNode> deque = new ArrayDeque<>();
        deque.addLast(root);
        while(!deque.isEmpty()) {
            int size = deque.size();
            List<Integer> res = new ArrayList<>();
            for(int i = 0; i < size; i++) {
                TreeNode node = deque.pollFirst();
                if(node.left != null) deque.addLast(node.left);
                if(node.right != null) deque.addLast(node.right);
                res.add(node.val);
            }
            result.add(res);
        }
        return result;
    }
}