package com.q0105;

import com.q0104_maximum_depth_of_binary_tree.TreeNode;

import java.util.HashMap;

import java.util.Map;

public class Solution2 {

//key:inorder[index],Value:index

private Map<Integer, Integer> map = new HashMap<>();

public TreeNode buildTree(int[] preorder, int[] inorder) {

if (preorder == null || inorder == null) {

return null;

}

map.clear();

for (int i = 0; i < inorder.length; i++) {

map.put(inorder[i], i);

}

return helper(preorder, 0, preorder.length - 1, inorder, 0, inorder.length - 1);

}

private TreeNode helper(int[] preorder, int pStart, int pEnd, int[] inorder, int iStart, int iEnd) {

if (pStart > pEnd || iStart > iEnd){

return null;

}

TreeNode node = new TreeNode(preorder[pStart]);

int rootPos = map.get(preorder[pStart]);

int leftNum = rootPos - iStart;

//前序:根左右

//中序:左根右

node.left = helper(preorder,pStart + 1,pStart + leftNum,inorder,iStart,iStart + leftNum);

node.right = helper(preorder,pStart + leftNum + 1,pEnd,inorder,rootPos + 1,iEnd);

return node;

}

}