【题目1】

给你一棵二叉树的根节点 root ，翻转这棵二叉树，并返回其根节点。

//===================递归===================    
public TreeNode invertTree(TreeNode root) {
        if(root == null)
            return root;
        invertTree(root.left);
        invertTree(root.right);
        TreeNode temp;
        temp = root.left;
        root.left = root.right;
        root.right = temp;
        return root;
    }

//===================迭代也一样=========
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null)
            return root;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode p = stack.pop();
            TreeNode temp;
            temp = p.left;
            p.left = p.right;
            p.right = temp;
            if(p.left != null)
                stack.push(p.left);
            if(p.right != null)
                stack.push(p.right);
        }
        return root;
    }
}
//=================层次也行===========

【题目2】

给你一个二叉树的根节点 root ， 检查它是否轴对称。

思路：两个指针，成对成对比较

class Solution {
    public boolean isSymmetric(TreeNode root) {
        return compare(root,root);
        
    }
    public boolean compare(TreeNode left,TreeNode right){
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(left);
        queue.offer(right);
        while(!queue.isEmpty()){
            left = queue.poll();
            right = queue.poll();
            if(left == null && right == null)  // 判断到了空子节点
                continue;
            if((left==null || right == null) ||  left.val != right.val)
                return false;
            queue.offer(left.left);
            queue.offer(right.right);
            queue.offer(left.right);
            queue.offer(right.left);
        }
        return true;
    }
}

class Solution {
    public boolean isSymmetric(TreeNode root) {
        return check(root, root);
    }

    public boolean check(TreeNode p, TreeNode q) {
        if (p == null && q == null) {
            return true;
        }
        if (p == null || q == null) {
            return false;
        }
        return p.val == q.val && check(p.left, q.right) && check(p.right, q.left);
    }
}

【题目3】二叉树最大深度  比较每个子树的深度，取最大深度

class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null)
            return 0;
        int leftDepth = maxDepth(root.left);
        int rightDepth = maxDepth(root.right);
        return (leftDepth>rightDepth?leftDepth:rightDepth)  + 1;
    }
}

【题目4】n叉树最大深度

class Solution {
    public int maxDepth(Node root) {
        if(root == null)
            return 0;
        int depth = 0;
        for(Node child : root.children){
            depth = Math.max(depth, maxDepth(child));
        }
        return depth  + 1;
    }
}