二叉树

BinaryTree

• 树的深度

• 形状

    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        height(root, 0, res);
        return res;
    }

    public void height(TreeNode node, int depth, List<Integer> res) {
        // 先读右子树，再读左子树，res 个数与二叉树的深度相同
        if (node == null) {
            return;
        }
        if (depth == res.size()) {
            res.add(node.val);
        }
        height(node.right, depth + 1, res);
        height(node.left, depth + 1, res);
    }

• 找公共祖先

• bfs

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