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原题链接在这里：https://leetcode.com/problems/serialize-and-deserialize-binary-tree/
把一个树serialize 和 deserialize. 递归方法是通过pre-order来serialize.
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例子就变成了"1,2,#,#,3,4,#,#,5,#,#". 然后deserialize先读入一个字符作为根节点，然后根节点的左右子节点递归调用deserializeHelper函数.
这里学到了StringTokenizer: StringTokenizer st = new StringTokenizer(data, ",")可以把data 用"," 隔开生成StringTokenizer.
StringTokenizer 的两个method: st.hasMoreTokens() 返回是否还有token 和 st.nextToken() 返回下一个string. StringTokenizer 很像 string.split().
但是他可以在中间过程中处理分出来的字符串。
AC Java:
1 public class Codec { 2 3 // Encodes a tree to a single string. 4 public String serialize(TreeNode root) { 5 StringBuilder sb = new StringBuilder(); 6 serializeHelper(root,sb); 7 return sb.toString(); 8 } 9 10 private void serializeHelper(TreeNode root, StringBuilder sb){ 11 if(root == null){ 12 sb.append("#,"); 13 return; 14 } 15 sb.append(root.val).append(","); 16 serializeHelper(root.left, sb); 17 serializeHelper(root.right, sb); 18 } 19 20 // Decodes your encoded data to tree. 21 public TreeNode deserialize(String data) { 22 StringTokenizer st = new StringTokenizer(data, ","); 23 return deserializeHelper(st); 24 } 25 26 private TreeNode deserializeHelper(StringTokenizer st){ 27 if(!st.hasMoreTokens()){ 28 return null; 29 } 30 String cur = st.nextToken(); 31 if(cur.equals("#")){ 32 return null; 33 } 34 TreeNode root = new TreeNode(Integer.valueOf(cur)); 35 root.left = deserializeHelper(st); 36 root.right = deserializeHelper(st); 37 return root; 38 } 39 }
Iteration 方法使用的BFS. 当处理serialize, queue不为空时，弹出节点cur, 若cur是null, 就在sb后面加上"#,"; 若不是null, 就在sb后面加上cur.val 和 ",".
处理deserialize时，先把string 用"," 格成string 数组。 如果是满树，那么左节点index就是2*i+1, 右节点index是2*i+2. 但这里不是满树。
需要同时记录当前i前面有几个null节点，用count存储，左节点index变成2*(i-count[i]) + 1, 右节点index变成2*(i-count[i]) + 2.
Note: 记住Queue 是 abstract class, 不能直接用来生成 que, 需要用LinkedList.
AC Java:
1 public class Codec { 2 3 // Encodes a tree to a single string. 4 public String serialize(TreeNode root) { 5 StringBuilder sb = new StringBuilder(); 6 Queue<TreeNode> que = new LinkedList<TreeNode>(); //error 7 que.offer(root); 8 while(!que.isEmpty()){ 9 TreeNode cur = que.poll(); 10 if(cur == null){ 11 sb.append("#,"); 12 }else{ 13 sb.append(cur.val).append(","); 14 que.offer(cur.left); 15 que.offer(cur.right); 16 } 17 } 18 return sb.toString(); 19 } 20 21 // Decodes your encoded data to tree. 22 public TreeNode deserialize(String data) { 23 String [] s = data.split(","); 24 int [] count = new int[s.length]; 25 TreeNode [] tns = new TreeNode[s.length]; 26 27 for(int i = 0; i<s.length; i++){ 28 if(i>0){ 29 count[i] = count[i-1]; 30 } 31 if(s[i].equals("#")){ 32 tns[i] = null; 33 count[i]++; 34 }else{ 35 tns[i] = new TreeNode(Integer.valueOf(s[i])); 36 } 37 } 38 39 for(int i = 0; i<s.length; i++){ 40 if(s[i].equals("#")){ 41 continue; 42 } 43 tns[i].left = tns[2 * (i-count[i]) + 1]; 44 tns[i].right = tns[2 * (i-count[i]) + 2]; 45 } 46 return tns[0]; 47 } 48 }
参考了这篇帖子：http://blog.csdn.net/ljiabin/article/details/49474445
LeetCode Serialize and Deserialize Binary Tree
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原文地址：http://www.cnblogs.com/Dylan-Java-NYC/p/4934908.html
