day21

今天是学习java的第21天，今天学习的是二叉树。

主要内容有，二叉树的建立，前、中、后序遍历，算二叉树的深度和节点数。

**树（Tree）**是n（n>=0)个结点的有限集。n=0时称为空树。在任意一颗非空树中：
1）有且仅有一个特定的称为根（Root）的结点；
2）当n>1时，其余结点可分为m(m>0)个互不相交的有限集T1、T2、…、Tn，其中每一个集合本身又是一棵树，并且称为根的子树。

结点子树的根结点为该结点的孩子结点。相应该结点称为孩子结点的双亲结点。

下图是今天所创建的二叉树：

树的遍历分为前序遍历，中序遍历，后序遍历。其实这些本质上是相同的，不同的是父亲节点的位置。前序遍历是先显示父亲节点，再读取孩子节点。中序则是父亲节点在中间，后序是父亲节点在孩子节点之后。

package datastructure;

public class BinaryCharTree {
    
    
    char value;

    
    BinaryCharTree leftChild;
    
    BinaryCharTree rightChild;

    
    public BinaryCharTree(char paraValue) {
        value = paraValue;
        leftChild = null;
        rightChild = null;
    } // Of the first construe

    public static BinaryCharTree manualConstructTree() {
        // step 1. Creat root.
        BinaryCharTree resultTree = new BinaryCharTree('a');

        // step 2. creat children and linked them.
        BinaryCharTree tempTreeB = new BinaryCharTree('b');
        BinaryCharTree tempTreeC = new BinaryCharTree('c');
        BinaryCharTree tempTreeD = new BinaryCharTree('d');
        BinaryCharTree tempTreeE = new BinaryCharTree('e');
        BinaryCharTree tempTreeF = new BinaryCharTree('f');
        BinaryCharTree tempTreeG = new BinaryCharTree('g');
        // link strat
        resultTree.leftChild = tempTreeB;
        resultTree.rightChild = tempTreeC;
        tempTreeB.rightChild = tempTreeD;
        tempTreeC.leftChild = tempTreeE;
        tempTreeD.leftChild = tempTreeF;
        tempTreeD.rightChild = tempTreeG;

        return resultTree;
    } // Of manualConstructTree

    
    public void preOrderVisit() {
        System.out.print(value + " ");

        if (leftChild != null) {
            leftChild.preOrderVisit();
        } // Of if

        if (rightChild != null) {
            rightChild.preOrderVisit();
        } // Of if
    } // Of preOrderVisit

    public void inOrderVisit() {
        if (leftChild != null) {
            leftChild.inOrderVisit();
        } // Of if

        System.out.print(value + " ");

        if (rightChild != null) {
            rightChild.inOrderVisit();
        } // Of if
    }

    public void postOrderVisit() {
        if (leftChild != null) {
            leftChild.postOrderVisit();
        } // Of if

        if (rightChild != null) {
            rightChild.postOrderVisit();
        } // Of if

        System.out.print(value + " ");
    }

    public int getDepth() {
        if ((leftChild == null) && (rightChild == null)) {
            return 1;
        } // Of if

        // check left
        int tempLeftDepth = 0;
        if (leftChild != null) {
            tempLeftDepth = leftChild.getDepth();
        } // Of if

        // Check right
        int tempRightDepth = 0;
        if (rightChild != null) {
            tempRightDepth = rightChild.getDepth();
        } // Of if

        if (tempLeftDepth >= tempRightDepth) {
            return tempLeftDepth + 1;
        } else {
            return tempRightDepth + 1;
        } // Of if
    } // Of getDepth

    public int getNumNodes() {
		if ((leftChild == null) && (rightChild == null)) {
			return 1;
		} // Of if

		int tempLeftNodes = 0;
		if (leftChild != null) {
			tempLeftNodes = leftChild.getNumNodes();
		} // Of if

		int tempRightNodes = 0;
		if (rightChild != null) {
			tempRightNodes = rightChild.getNumNodes();
		} // Of if

		return tempLeftNodes + tempRightNodes + 1;
	}// Of getNumNodes

	
	public static void main(String args[]) {
		BinaryCharTree tempTree = manualConstructTree();
		System.out.println("rnPreorder visit:");
		tempTree.preOrderVisit();
		System.out.println("rnIn-order visit:");
		tempTree.inOrderVisit();
		System.out.println("rnPost-order visit:");
		tempTree.postOrderVisit();

		System.out.println("rnrnThe depth is: " + tempTree.getDepth());
		System.out.println("The number of nodes is: " + tempTree.getNumNodes());
	}// Of main
}

运行结果：