From 7161f1ae4ba846e0a4edccab5273a1534ca85f36 Mon Sep 17 00:00:00 2001 From: ShusenTang Date: Fri, 7 Feb 2020 20:25:10 +0800 Subject: [PATCH] add 297 --- README.md | 1 + .... Serialize and Deserialize Binary Tree.md | 135 ++++++++++++++++++ 2 files changed, 136 insertions(+) create mode 100644 solutions/297. Serialize and Deserialize Binary Tree.md diff --git a/README.md b/README.md index 924f5dd..31c4f0e 100644 --- a/README.md +++ b/README.md @@ -195,6 +195,7 @@ My LeetCode solutions with Chinese explanation. 我的LeetCode中文题解。 | 289 |[Game of Life](https://leetcode.com/problems/game-of-life/)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/289.%20Game%20of%20Life.md)|Medium| | | 290 |[Word Pattern](https://leetcode.com/problems/word-pattern)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/290.%20Word%20Pattern.md)|Easy| | | 292 |[Nim Game](https://leetcode.com/problems/nim-game)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/292.%20Nim%20Game.md)|Easy| | +| 297 |[Serialize and Deserialize Binary Tree](https://leetcode.com/problems/serialize-and-deserialize-binary-tree/)|[C++](solutions/297.%20Serialize%20and%20Deserialize%20Binary%20Tree.md)|Hard| | | 300 |[Longest Increasing Subsequence](https://leetcode.com/problems/longest-increasing-subsequence/)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/300.%20Longest%20Increasing%20Subsequence.md)|Medium| | | 303 |[Range Sum Query - Immutable](https://leetcode.com/problems/range-sum-query-immutable)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/303.%20Range%20Sum%20Query%20-%20Immutable.md)|Easy| | | 304 |[Range Sum Query 2D - Immutable](https://leetcode.com/problems/range-sum-query-2d-immutable/)|[C++](https://github.com/ShusenTang/LeetCode/blob/master/solutions/304.%20Range%20Sum%20Query%202D%20-%20Immutable.md)|Medium| | diff --git a/solutions/297. Serialize and Deserialize Binary Tree.md b/solutions/297. Serialize and Deserialize Binary Tree.md new file mode 100644 index 0000000..81b39cd --- /dev/null +++ b/solutions/297. Serialize and Deserialize Binary Tree.md @@ -0,0 +1,135 @@ +# [297. Serialize and Deserialize Binary Tree](https://leetcode.com/problems/serialize-and-deserialize-binary-tree/) + +# 思路 +对二叉树进行序列化和去序列化的操作。 + +## 思路一 +本质上就是遍历二叉树，那我们考虑用递归前序遍历二叉树。 +* 序列化： + * 如果节点为空则添加"# "（用空格作为分隔符，这样就可以用字符串流）； + * 否则添加节点值（要紧跟一个空格）再递归进入左右孩子。 + +* 去序列化： + * 如果当前字符为"#"，则返回空； + * 否则根据字符串的值新建节点，然后递归求其左右孩子。 + +我们可以用输入/输出字符串流`istringstream`/`ostringstream`来进行处理，详见代码。 + +## 思路二 +我们还可以用非递归的层序遍历，这样就类似LeetCode使用的方法了。 + +* 序列化： + * 将根节点入队列，循环直到队列为空：出队首元素，若其为空则添加"# "；否则添加节点值（要紧跟一个空格），另外还要将其左右孩子入队（即使孩子为空）。 + +* 去序列化： + * 先处理得到根节点，然后将根节点入队，循环直到队列空：处理得到队首节点的左孩子，即将队首元素的左指针指向当前处理得到的节点，若这个左孩子不为空还要将其入队；再用类似的方法处理得到队首节点的右孩子；最后将队首元素出队。 + +同样用输入/输出字符串流`istringstream`/`ostringstream`来进行处理，详见代码。 + +本质都是遍历二叉树，所以两个思路时空复杂度均为O(n) + +# C++ +## 思路一 +``` C++ +class Codec { +private: + void serialize(TreeNode* root, ostringstream &out){ + if(!root) out << "# "; + else{ + out << root -> val << " "; + serialize(root -> left, out); + serialize(root -> right, out); + } + } + TreeNode* deserialize(istringstream &in){ + string val; + in >> val; + if(val == "#") return NULL; + + TreeNode *root = new TreeNode(stoi(val)); + root -> left = deserialize(in); + root -> right = deserialize(in); + return root; + } + +public: + + // Encodes a tree to a single string. + string serialize(TreeNode* root) { + if(!root) return ""; + ostringstream out; + serialize(root, out); + return out.str(); + } + + // Decodes your encoded data to tree. + TreeNode* deserialize(string data) { + if(!data.size()) return NULL; + + istringstream in(data); + return deserialize(in); + } +}; +``` + +## 思路二 +``` C++ +class Codec { +public: + // Encodes a tree to a single string. + string serialize(TreeNode* root) { + if(!root) return ""; + ostringstream out; + + TreeNode *p; + queueq; + q.push(root); + + while(!q.empty()){ + p = q.front(); q.pop(); + if(!p) out << "# "; + else{ + out << p -> val << " "; + q.push(p -> left); + q.push(p -> right); + } + } + return out.str(); + } + + // Decodes your encoded data to tree. + TreeNode* deserialize(string data) { + int len = data.size(); + if(!len) return NULL; + + queueq; + TreeNode *p = NULL, *root = NULL; + + istringstream in(data); + string val; + + // 根节点 + in >> val; + p = new TreeNode(stoi(val)); root = p; + q.push(p); + + while(!q.empty()){ + in >> val; // 左孩子 + if(val != "#"){ + p = new TreeNode(stoi(val)); + q.push(p); + q.front() -> left = p; + } + + in >> val; // 右孩子 + if(val != "#"){ + p = new TreeNode(stoi(val)); + q.push(p); + q.front() -> right = p; + } + q.pop(); + } + return root; + } +}; +``` \ No newline at end of file