二叉树1

144.二叉树的前序遍历

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root==null) return res;
        Stack<TreeNode> stack = new Stack<>();
        stack.add(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;
    }
}

102.二叉树的层次遍历

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if(root==null) return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            for(int i=0;i<size;i++){
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left!=null){
                    queue.add(node.left);
                }
                if(node.right!=null){
                    queue.add(node.right);
                }
            }
            res.add(list);
        }
        return res;
    }
}