LeetCode226. 翻转二叉树(day0012_1)

给你一棵二叉树的根节点 root ，翻转这棵二叉树，并返回其根节点。

示例 1：

 输入：root = [4,2,7,1,3,6,9]
 输出：[4,7,2,9,6,3,1]

示例 2：

 输入：root = [2,1,3]
 输出：[2,3,1]

示例 3：

 输入：root = []
 输出：[]

提示：

• 树中节点数目范围在 [0, 100] 内

• -100 <= Node.val <= 100

前序迭代器反转：

/**
 * 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) {
        stack<TreeNode*>st;
        if(root==NULL)return root;
        st.push(root);
        while(!st.empty()){
            TreeNode* node=st.top();
            st.pop();
            swap(node->left,node->right);
            if(node->right)st.push(node->right);
            if(node->left)st.push(node->left);
        }
        return root;
    }
};

递归反转：

/**
 * 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==NULL)return root;
        swap(root->left,root->right);
        invertTree(root->left);
        invertTree(root->right);
        return root;
    }
};

层序遍历反转：

/**
 * 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) {
        queue<TreeNode*>que;
        if(root!=NULL)que.push(root);
        while(!que.empty()){
            int size=que.size();
            for(int i=0;i<size;i++){
                TreeNode* node=que.front();
                que.pop();
                swap(node->left,node->right);
                if(node->left)que.push(node->left);
                if(node->right)que.push(node->right);
            }
        }
        return root;
    }
};