LeetCode 226翻转二叉树

LeetCode 226翻转二叉树

• 题目简述：翻转一棵二叉树。
• 输入：[4, 2, 7, 1, 3, 6, 9] 输出：[4, 7, 2, 9, 6, 3, 1]
• 思路：递归遍历整棵二叉树，遍历到每个节点时交换左右儿子。时间复杂度O(n)

class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(!root) return 0;
        swap(root->left, root->right);
        invertTree(root->left);
        invertTree(root->right);
        return root;
    }
};

• 思路：利用栈或者队列迭代

//栈写法
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(!root) return 0;
        stack<TreeNode*> s;
        s.push(root);
        while(s.size())
        {
            TreeNode* cur = s.top();
            //swap(cur->left, cur->right);//等价于下面三行
            TreeNode* tmp = cur->right;
            cur->right = cur->left;
            cur->left = tmp;
            s.pop();
            if(cur->left) s.push(cur->left);
            if(cur->right) s.push(cur->right);
        }
        return root;
    }
};
//队列写法
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(!root) return 0;
        queue<TreeNode*> q;
        q.push(root);
        while(q.size())
        {
            TreeNode* cur = q.front();
            swap(cur->left, cur->right);
            q.pop();
            if(cur->left) q.push(cur->left);
            if(cur->right) q.push(cur->right);
        }
        return root;
    }
};