Leetcode 101. Symmetric Tree

文章作者：Tyan
博客：noahsnail.com  |  CSDN  |  简书

1. Description

2. Solution

• Recursive

/**
 * 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) {
        if(root == NULL) {
            return true;
        }
        return isMirror(root->left, root->right);
    }
    
    bool isMirror(TreeNode* left, TreeNode* right) {
        if(left == NULL && right == NULL) {
            return true;
        }
        if(left == NULL || right == NULL) {
            return false;
        }
        return left->val == right->val && isMirror(left->left, right->right) && isMirror(left->right, right->left);
    }
};

• Iterative

  /**
   * 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) {
          if(root == NULL) {
              return true;
          }
          queue<TreeNode*> nodes;
          nodes.push(root);
          nodes.push(root);
          while(!nodes.empty()) {
              TreeNode* left = nodes.front();
              nodes.pop();
              TreeNode* right = nodes.front();
              nodes.pop();
              if(left == NULL && right == NULL) {
                  continue;
              }
              if(left == NULL || right == NULL) {
                  return false;
              }
              if(left->val == right->val) {
                  nodes.push(left->left);
                  nodes.push(right->right);
                  nodes.push(left->right);
                  nodes.push(right->left);
              }
              else {
                  return false;
              }
          }
          return true;
      }
  };

Reference

1. https://leetcode.com/problems/symmetric-tree/description/