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Leetcode0105.java
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Leetcode0105.java
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package com.dezhonger.y2020.m01.d24;
import com.dezhonger.y2019.TreeNode;
/**
* Created by dezhonger on 2020/01/24
*
* @author dezhonger
* @since 2020/01/24
*/
public class Leetcode0105 {
public TreeNode buildTree(int[] preorder, int[] inorder) {
int len = preorder.length;
return help(0, len - 1, preorder, 0, len - 1, inorder);
}
private TreeNode help(int s1, int e1, int[] preorder, int s2, int e2, int[] inorder) {
//important
if (s1 > e1) return null;
if (s2 > e2) return null;
if (s1 > preorder.length - 1) return null;
TreeNode root = new TreeNode(preorder[s1]);
int mid = -1;
for (int i = s2; i <= e2; i++) {
if (inorder[i] == root.val) {
mid = i;
break;
}
}
root.left = help(s1 + 1, e1, preorder, s2, mid - 1, inorder);
root.right = help(s1 + 1 + mid - s2, e1, preorder, mid + 1, e2, inorder);
return root;
}
}