LeetCode 0101 Symmetric Tree【二叉树，树的对称，递归】

题目链接 LeetCode | 力扣

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3

Follow up: Solve it both recursively and iteratively.

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题意

判断是否是镜像二叉树

思路1

• 均为叶节点时满足
• 一个节点为叶节点另一个不是时不满足
• 非叶节点需要满足值相等，左子树和右子树相等，右子树和左子树相等

代码1

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        return isMirror(root, root);
        
        // 等价于
        // if(root == NULL) return true;
        // else return isMirror(root->left, root->right);
    }
    
    bool isMirror(TreeNode* t1, TreeNode* t2) {
        if(t1 == NULL && t2 == NULL) return true; // 叶结点
        if(t1 == NULL || t2 == NULL) return false; // 不符合
        return t1->val==t2->val && isMirror(t1->left, t2->right) && isMirror(t1->right, t2->left);
    }
};