https://leetcode-cn.com/problems/dui-cheng-de-er-cha-shu-lcof/
class Solution { public boolean isSymmetric(TreeNode root) { return root == null ? true : recur(root.left, root.right); } boolean recur(TreeNode L, TreeNode R){ //如果两边都是空的,true if(L == null && R == null) return true; //任何一个不满足,false if(L == null || R == null || L.val != R.val) return false; //这一层满足,递归下一层 return recur(L.left, R.right) && recur(L.right, R.left); } }