【剑指offer】面试题37：序列化二叉树

题目

请实现两个函数，分别用来序列化和反序列化二叉树

思路

使用前序遍历来序列化二叉树，当碰到子节点为null的时候，用特殊字符’$’来表示。

ps：也可以使用后序遍历（左->右->根）来做。但是不能使用中序遍历来做（左->根->右），因为使用中序遍历无法确定这棵二叉树。

代码

public class _37_SerializeBinaryTrees {

public static class TreeNode {
public int val = 0;
public TreeNode left = null;
public TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}

/**
* 序列化
*/
public String Serialize(TreeNode root) {
if(root == null)
return "$";
StringBuilder sb = new StringBuilder();
Serialize(root, sb);
return sb.toString();
}

private void Serialize(TreeNode root, StringBuilder sb) {
if(root == null) {
sb.append("$");
return;
}
sb.append(root.val);
Serialize(root.left, sb.append(","));
Serialize(root.right, sb.append(","));
}

private int index;
/**
* 反序列化
*/
public TreeNode Deserialize(String str) {
if(str == null || str.trim().length() == 0)
return null;
String[] nums = str.split(",");
index = 0;
return Deserialize(nums);
}

public TreeNode Deserialize(String[] nums) {
if(nums[index].equals("$")) {
++index;
return null;
}
TreeNode node = new TreeNode(Integer.valueOf(nums[index++]));
node.left = Deserialize(nums);
node.right = Deserialize(nums);
return node;
}
}

测试

public class _37_Test {

public static void main(String[] args) {
test1();
test2();
test3();
}

/**
* 功能测试
* 1.序列化
*     1
*    / \
*   2   3
*  /   / \
* 4   5   6
*
* 2.反序列化
*/
private static void test1() {
TreeNode node1 = new TreeNode(1);
TreeNode node2 = new TreeNode(2);
TreeNode node3 = new TreeNode(3);
TreeNode node4 = new TreeNode(4);
TreeNode node5 = new TreeNode(5);
TreeNode node6 = new TreeNode(6);
node1.left = node2;
node1.right = node3;
node2.left = node4;
node3.left = node5;
node3.right = node6;
_37_SerializeBinaryTrees sbt = new _37_SerializeBinaryTrees();
String s = sbt.Serialize(node1);
// 1,2,4,$,$,$,3,5,$,$,6,$,$
System.out.println(s);

TreeNode newRoot = sbt.Deserialize(s);
String s2 = sbt.Serialize(newRoot);
// 1,2,4,$,$,$,3,5,$,$,6,$,$
System.out.println(s);
}

/**
* 边界测试
* 1.只有左子节点
* 2.只有右子节点
* 3.只有一个节点
*/
private static void test2() {
_37_SerializeBinaryTrees sbt = new _37_SerializeBinaryTrees();
TreeNode root1 = sbt.Deserialize("1,2,3,$,$,$,$");
System.out.println(sbt.Serialize(root1));

TreeNode root2 = sbt.Deserialize("1,$,2,$,3,$,$");
System.out.println(sbt.Serialize(root2));

TreeNode root3 = sbt.Deserialize("1,$,$");
System.out.println(sbt.Serialize(root3));
}

/**
* 极端测试
* 1.反序列化时输入空字符串
* 2.序列化时输入null
*/
private static void test3() {
_37_SerializeBinaryTrees sbt = new _37_SerializeBinaryTrees();
TreeNode root = sbt.Deserialize("");
System.out.println(sbt.Serialize(root));
}
}