二叉樹根據前序中序重建

題目描述：輸入某二叉樹的前序遍歷和中序遍歷的結果，請重建出該二叉樹。假設輸入的前序遍歷和中序遍歷的結果中都不含重複的數字。例如輸入前序遍歷序列{1,2,4,7,3,5,6,8}和中序遍歷序列{4,7,2,1,5,3,8,6}，則重建二叉樹並返回。

 

 

 

java代碼：

class TreeNode {
	int val = 0;
	TreeNode left = null;
	TreeNode right = null;

	public TreeNode(int val) {
		this.val = val;
	}
}

 

public class RebuildTreeByPreAndMid {
	public TreeNode reConstructBinaryTree(int [] pre,int [] in) {
        if(pre==null||in==null||pre.length==0||in.length==0){
        	return null;
        }
		return f(pre,in,0,pre.length-1,0,in.length-1);
    }
	
	public TreeNode f(int [] pre,int [] mid,int pStart,int pEnd,int mStart,int mEnd){
		if(pStart>pEnd||mStart>mEnd){
			return null;
		}
		TreeNode p=new TreeNode(pre[pStart]);
		int pos=-1;	// 找出根節點所在位置，從而確定左右子孫的範圍
		for(int i=mStart;i<=mEnd;i++){
			if(mid[i]==pre[pStart]){
				pos=i;
				break;
			}
		}
		p.left=f(pre,mid,pStart+1,pEnd,mStart,pos-1);
		p.right=f(pre,mid,pStart+pos-mStart+1,pEnd,pos+1,mEnd);
		return p;
	}
	
	// 前序遍歷檢驗
	public void preOrder(TreeNode head){
		if(head!=null){
			System.out.print(head.val+" ");
			preOrder(head.left);
			preOrder(head.right);
		}else{
			return;
		}
	}
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		RebuildTreeByPreAndMid test= new RebuildTreeByPreAndMid();
		int pre[]={1,2,4,7,3,5,6,8};
		int in[]={4,7,2,1,5,3,8,6};
		TreeNode head=test.reConstructBinaryTree(pre, in);
		test.preOrder(head);
	}
}