Java二叉樹的遍歷

二叉樹

二叉樹的是常見的數據結構，二叉樹的遍歷有三種，分別爲前序遍歷、中序遍歷和後序遍歷。下面看看使用Java實現二叉樹。

1. 前序遍歷：先遍歷根節點，再遍歷左子樹，最後遍歷右子樹
2. 中序遍歷：再遍歷左子，樹先遍歷根節點，最後遍歷右子樹
3. 後序遍歷：再遍歷左子，最後遍歷右子樹，樹先遍歷根節點

如下圖爲例

前序遍歷：ABDEGCF

中序遍歷：DBGEACF

後序遍歷：DGEBFCA

二叉樹的實現

JAVA實現須要創建三個類

1. TreeNode.java
2. TreeCreator.java
3. TreeTreavervsal.java

下面是Java代碼

TreeNode.java

package Tree;

public class TreeNode {
 
	private final char value;
	private TreeNode left;
	private TreeNode right;
	
	public TreeNode(char value) {
		this.value=value;
		this.left=null;
		this.right=null;
		
	}
	public char getValue() {
		return value;
	}
	
	public TreeNode getLeft() {
		return left;
	}
	public void setLefg(TreeNode left) {
		this.left=left;
	}
	public TreeNode getRight() {
		return right;
	}
	public void setRight(TreeNode right) {
		this.right=right;
	}
}

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2.TreeCreator.java

package Tree;

public class TreeCreator {

	public TreeNode creatorSampleTree() {
		TreeNode root =new TreeNode('A');
		root.setLefg(new TreeNode('B'));
		root.getLeft().setLefg(new TreeNode('D'));
		root.getLeft().setRight(new TreeNode('E'));
		root.getLeft().getRight().setLefg(new TreeNode('G'));
		root.setRight(new TreeNode('C'));
		root.getRight().setRight(new TreeNode('F'));
		return root;
		
	}
	
}

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3.TreeTreavervsal.java

package Tree;

import javax.sql.rowset.spi.SyncProvider;

public class TreeTraversal {

	public void preOrder(TreeNode root) {
		if(root == null) {
			return;
		}
		System.out.print(root.getValue());
		preOrder(root.getLeft());
		preOrder(root.getRight());
	}
	public void inOrder(TreeNode root) {
		if(root == null) {
			return;
		}
		inOrder(root.getLeft());
		System.out.print(root.getValue());
		inOrder(root.getRight());
	}
	public void postOrder(TreeNode root) {
		if(root == null) {
			return;
		}
	
		postOrder(root.getLeft());
		
		postOrder(root.getRight());
		System.out.print(root.getValue());
	}
	
	
	public static void main(String[] args) {
		TreeCreator creator=new TreeCreator();
		TreeTraversal treaversal =new TreeTraversal();
		
		TreeNode sampleTree=creator.creatorSampleTree();
		treaversal.preOrder(sampleTree);
		System.out.println();
		treaversal.inOrder(sampleTree);
		System.out.println();
		treaversal.postOrder(sampleTree);
		
	}
}

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已知前序遍歷和中序遍歷求後序遍歷

例如：

前序遍歷：ABDEGCF

中序遍歷：DBGEACF

基本思想：根據前序遍歷的特色得出A爲根節點，那麼中心遍歷的A也爲根節點，那麼根據中序遍歷特色A左邊的爲A的左子樹，即DBGE爲左子樹，CF爲右子樹數，以此遞歸，求出樹，而後最終得出後續遍歷。

Java代碼

public TreeNode creatorTree(String preOrder,String inOrder) {
		
		if(preOrder.equals("")) {
			return null;
		}
		char rootValue=preOrder.charAt(0);
		int rootIndex =inOrder.indexOf(rootValue);
		TreeNode root=new TreeNode(rootValue);
		
		root.setLefg(
				creatorTree(
						preOrder.substring(1, rootIndex+1),
						inOrder.substring(0, rootIndex)));
				
		root.setRight(creatorTree(
				preOrder.substring(1+rootIndex),
				inOrder.substring(1+rootIndex)
				));		
		return root;
		
	
}
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