反转二叉树

Invert Binary Tree

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• 题目来源：
  leetcode 226

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名人轶事：

该题曾是goole的一道面试题；当年Max Howell （Homebrew的作者）面试时没有做出该题而被 Google 拒绝。

goole给Max Howell的回复是：
Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

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算法实现

TreeNode* invertTree(TreeNode* root) {  //先序遍历
    if (root==NULL) {
        return root;
    }
    else{
        TreeNode * temp = root->left;
        root->left = root->right;
        root->right = temp;
        root->left = invertTree(root->left);
        root->right = invertTree(root->right);
        return root;
    }
}

TreeNode* invertTree(TreeNode* root) {  //中序遍历
    if (root==NULL) {
        return root;
    }
    else{

        root->left = invertTree(root->left);
        TreeNode * temp = root->left;
        root->left = root->right;
        root->right = temp;
        root->left = invertTree(root->left);
        return root;
    }
}

TreeNode* invertTree(TreeNode* root) {  //后序遍历
    if (root==NULL) {
        return root;
    }
    else{
        TreeNode * left = invertTree(root->left);
        TreeNode * right = invertTree(root->right);
        root->left = right;
        root->right = left;
        return root;
    }
}

TreeNode* invertTree(TreeNode* root) {  //层序遍历
    if (root==NULL) {
        return root;
    }
    else{
        queue<TreeNode *> Q;
        Q.push(root);
        while (!Q.empty()) {
            TreeNode * top = Q.front();
            Q.pop();
            if(top->right!=NULL) Q.push(top->right);
            if(top->left!=NULL) Q.push(top->left);
            TreeNode * temp = top->left;
            top->left = top->right;
            top->right = temp;
        }
        return root;

    }
}

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算法分析

• 先中后序遍历：
  

• 层次遍历：