Leetcode_ Binary Tree to Linked List
2014-02-25 08:46
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题目
Given a binary tree, flatten it to a linked list in-place.For example,
Given
1 / \ 2 5 / \ \ 3 4 6
给定二叉树,要求把它变为链型,节点顺序是先序
The flattened tree should look like:
1 \ 2 \ 3 \ 4 \ 5 \ 6
思路
怎样递归呢?能明显看出,变为线性后的序列是原树的先序遍历,直观感觉上,应该递归的方式应该和先序遍历的递归方式有很大的联系,先序线索化二叉树,更准确的说我们这样来分息递归结构,每次访问一个节点的时候 ,我们必须要知道它的上一个访问的节点是什么,这样才能把上一个节点的右孩子指向当前访问的跟节点,所以我们用pre来保存,这样自己模拟的走两遍,就会发现递归结构和先序一样,但需要注意的是因为我们是把前驱节点的右孩子指向当前节点,所以 如果我们照搬先序遍历的话,当左子树访问完了,访问右子树指针已经变了,所以我们在进行右子树递归前应该用递归左子树前保存的指针,之前没有AC,后来看disscuss,发现自己没有把左子树赋值为NULL。
代码
class Solution { public: void flatten(TreeNode *root) { TreeNode *p,*pre; TreeNode *dump=new TreeNode(-1); if (root==NULL) return; flatternHelper(root,dump); /*while(root!=NULL){ cout<<root->val<<endl; root=root->right; }*/ } void flatternHelper(TreeNode *root,TreeNode* &pre) { if (root!=NULL) { //cout<<root->val<<endl; pre->right = root; pre->left = NULL;//this line can't be miss! pre = root; TreeNode *p = root->right; flatternHelper(root->left,pre); flatternHelper(p,pre); } } };
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