LC226.翻转二叉树

思路1：DFS

• 利用递归的结构进行翻转

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if (!root) return nullptr;
        TreeNode* right = invertTree(root->left);
        TreeNode* left = invertTree(root->right);
        root->left = left;
        root->right = right; 
        return root;
    }
};

思路2：BFS

• 层次遍历二叉树
• 对于每层的每个节点，都有左右孩子，那么交换每个节点的左右孩子，就相当于把整个二叉树翻转了一遍。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if (!root) {
            return root;
        }
        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            TreeNode* cur_node = q.front();
            q.pop();
            if (cur_node->left || cur_node->right) {
                // exchange
                TreeNode* temp = cur_node->left;
                cur_node->left = cur_node->right;
                cur_node->right = temp;
            }
            if (cur_node->left) {q.push(cur_node->left);}
            if (cur_node->right) {q.push(cur_node->right);}
        }
        return root;
    }
};