Leetcode sword finger offer 28. symmetric binary tree

Title Description ：

Please implement a function , Used to judge whether a binary tree is symmetrical . If a binary tree is the same as its mirror image , So it's symmetrical .

for example , Binary tree [1,2,2,3,4,4,3] It's symmetrical .

    1
   / \
  2   2
 / \ / \
3  4 4  3
But the next one [1,2,2,null,3,null,3] It's not mirror symmetric :

    1
   / \
  2   2
   \   \
   3    3

Examples ：

Example 1：

Input ：root = [1,2,2,3,4,4,3]
Output ：true

Example 2：

Input ：root = [1,2,2,null,3,null,3]
Output ：false

Limit ：

0 <= Number of nodes <= 1000

Their thinking ：

    Symmetric binary tree definition ： For trees Any two symmetric nodes L and R , There must be ：
        L.val=R.val ： That is, the values of these two symmetric nodes are equal .
        L.left.val=R.right.val ： namely L Of The left child node and R Of The right child node symmetry ;
        L.right.val=R.left.val： namely L Of The right child node and R Of The left child node symmetry .
    According to the above rule , Consider recursion from top to bottom , Judge whether each pair of nodes is symmetrical , So as to judge whether the tree is a symmetric binary tree .

Java Program ：

class Solution {
    public boolean isSymmetric(TreeNode root) {
        return root == null ? true : recur(root.left, root.right);
    }
    boolean recur(TreeNode L, TreeNode R) {
        if(L == null && R == null) return true;
        if(L == null || R == null || L.val != R.val) return false;
        return recur(L.left, R.right) && recur(L.right, R.left);
    }
}