leetcode Construct Binary Tree from Inorder and Postorder Traversal 难度系数3 3.38

Question:

Given inorder and postorder traversal of a tree, construct the binary tree.

Note:

You may assume that duplicates do not exist in the tree.

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

private TreeNode subBuild(int[] inorder, int instart, int inend, int[] postorder,
int poststart, int postend) {
int root = postorder[postend];
TreeNode tn = new TreeNode(root);
if(instart==inend) return tn;
int index = instart;
while(inorder[index]!=root){
index++;
}
int leftlen = index-instart-1;
int rightlen = inend-index-1;
if(index!=instart){
tn.left = subBuild(inorder, instart, index-1, postorder, poststart, poststart+leftlen);
}else {
tn.left = null;
}
if(index!=inend){
tn.right = subBuild(inorder, index+1, inend, postorder, postend-1-rightlen, postend-1);
}else {
tn.right = null;
}
return tn;
}
}