根据前序遍历和中序遍历结果构造二叉树

根据 前序遍历 和 中序遍历 结果 构造相应的二叉树

基本思路：

1.在前序遍历结果中查找子树的根

2.在中序遍历结果中查找左右子树的结点的数量

3. 递归地执行步骤1和2 构造各层子树

package com.cupid.algorithm.tree;

public class ConstructBinaryTree {
/**
* Construct a binary tree based on the pre-order traversal result and in-order traversal result
*
* @param preOrder The array which stores the pre-order traversal result
* @param inOrder The array which stores the in-order traversal result
* @param preBegin This index always refers to the location of roots of sub-trees
* @param preEnd This index always refers to where the end of a sub-tree is
* @param preLength How many elements of a sub-tree
* @param inBegin This index refers to where a left sub-tree begins
* @return
*/
public static BinaryTree construct(int[] preOrder,int[] inOrder,int preBegin,int preEnd,int preLength,int inBegin){
// If left or right sub-tree is empty
// simply return that empty tree.
BinaryTree myTree = new BinaryTree(null);
if(preBegin>preEnd){
return myTree;
}
// Plant a tree, specifying its root.
myTree.root = new BinaryTreeNode(preOrder[preBegin]);
// When preBegin equals preEnd,
// there is only one element(the root) in a tree,
// simply return that tree.
if(preBegin == preEnd){
return myTree;
}
int i = inBegin;
int leftCount = 0;
while(inOrder[i] != preOrder[preBegin]){
leftCount++;
i++;
}
int rightCount = preLength - leftCount -1;
myTree.root.left = construct(preOrder, inOrder, preBegin+1, preBegin + leftCount,leftCount,inBegin).root;
myTree.root.right = construct(preOrder,inOrder, preBegin + leftCount +1, preBegin + leftCount+ rightCount,rightCount,i+1).root;
return myTree;
}

public static void main(String[] args) {
int[] preOrder = new int[]{1,2,3,4,5};
int[] inOrder = new int[]{2,1,4,3,5};
//int[] preOrder = new int[]{12,13,9,4,2,8,1,6,7,3,15,5,10};
// int[] inOrder = new int[]{4,9,2,13,8,1,12,3,7,15,6,5,10};
BinaryTree myTree = construct(preOrder, inOrder,0,preOrder.length-1,preOrder.length,0);
myTree.preOrder(myTree.root);
System.out.println();
myTree.inOrder(myTree.root);
}

}

class BinaryTree{

public BinaryTreeNode root = null;
public BinaryTree(BinaryTreeNode root){
this.root = root;
}
public void inOrder(BinaryTreeNode root){
if(root!=null){
inOrder(root.left);
System.out.print(root.key + "-");
inOrder(root.right);
}
}

public void preOrder(BinaryTreeNode root){
if(root!=null){
System.out.print(root.key + "-");
preOrder(root.left);
preOrder(root.right);
}
}
}

class BinaryTreeNode{
public int key;
public BinaryTreeNode parent = null;
public BinaryTreeNode left = null;
public BinaryTreeNode right = null;

public BinaryTreeNode(int key){
this.key = key;
}
}