105. 从前序与中序遍历序列构造二叉树

105. 从前序与中序遍历序列构造二叉树

根据一棵树的前序遍历与中序遍历构造二叉树。

例如，给出

前序遍历 preorder = [3,9,20,15,7]
中序遍历 inorder = [9,3,15,20,7]
返回如下的二叉树：

    3
   / \
  9  20
    /  \
   15   7

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        return buildTree(preorder,0,preorder.length-1,inorder,0,inorder.length-1);
    }

    public TreeNode buildTree(int[] preorder, int preLeft,int preRight, int[] inorder,int inLeft,int inRight){
        //递归终止条件
        if(preLeft>preRight||inLeft>inRight){
            return null;
        }
        //获取当前节点的值
        int value=preorder[preLeft];
        //新建节点
        TreeNode node=new TreeNode(value);
        //从中序数组左边界开始，找到中序遍历中根节点位置下标
        int index=inLeft;
        while(inorder[index]!=value){
            index++;
        }
        //index-inLeft：左子树节点个数
        node.left=buildTree(preorder,preLeft+1,preLeft+index-inLeft,inorder,inLeft,index-1);
        node.right=buildTree(preorder,preLeft+index-inLeft+1,preRight,inorder,index+1,inRight);
        return node;
    }

}

参考题解：https://leetcode-cn.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/solution/qian-xu-bian-li-python-dai-ma-java-dai-ma-by-liwei/