leetcode Construct Binary Tree from Preorder and Inorder Traversal

题目链接

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


        TreeNode root=new TreeNode(preorder[0]);
        build(root, inorder, 0, inorder.length-1, preorder, 0, preorder.length-1);

        return root;
    }


    public void build(TreeNode root,int[] inorder, int ins,int ine,int[] preorder,int pres,int pree)
    {
        int parentIndexInMiddle=ins;

        while(inorder[parentIndexInMiddle]!=root.val)
        {
            parentIndexInMiddle++;
        }

        int leftCount=parentIndexInMiddle-ins;



        if(leftCount>0)
        {
            int leftRootIndexInPre=pres+1;
            root.left=new TreeNode(preorder[leftRootIndexInPre]);
            build(root.left, inorder, ins, parentIndexInMiddle-1, preorder, leftRootIndexInPre, pres+1+leftCount-1);
        }

        int rightCount=ine-parentIndexInMiddle;
        if(rightCount>0)
        {
            int rightRootIndexInpre=pree-rightCount+1;
            root.right=new TreeNode(preorder[rightRootIndexInpre]);
            build(root.right, inorder, parentIndexInMiddle+1, ine, preorder, rightRootIndexInpre, pree);
        }
    }
}