对称问题

101. 对称二叉树

一种递归技巧，两个变量向下递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return dfs(root.left, root.right);
    }
    boolean dfs(TreeNode l, TreeNode r) {
        if (l == null && r == null) return true;
        if (l == null || r == null) return false;
        if (l.val != r.val) return false;
        boolean res = true;
        res &= dfs(l.left, r.right);
        res &= dfs(l.right, r.left);
        return res;
    }
}