【LeetCode】105.Construct Binary Tree from Preorder and Inorder Traversal(Medium)解题报告

【LeetCode】105.Construct Binary Tree from Preorder and Inorder Traversal(Medium)解题报告

题目地址：https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/description/
题目描述：

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

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

For example, given

preorder = [3,9,20,15,7]
inorder = [9,3,15,20,7]
Return the following binary tree:

    3
   / \
  9  20
    /  \
   15   7

Solutions:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 time : O(n)
 space : O(n)
 */
class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        return helper(0,0,inorder.length-1,preorder,inorder);
    }
    public TreeNode helper(int preStart,int inStart,int inEnd,int[] preorder,int[] inorder){
        if(preStart>preorder.length-1 || inStart>inEnd){
            return null;
        }
        TreeNode root = new TreeNode(preorder[preStart]);
        int inIndex = 0;
        for(int i=inStart ; i <= inEnd ; i++){
            if(inorder[i] == root.val){
                inIndex = i;
            }
        }
        root.left = helper(preStart+1,inStart,inIndex-1,preorder,inorder);
        root.right = helper(preStart+inIndex-inStart+1,inIndex+1,inEnd,preorder,inorder);
        return root;
    }
}

Date：2018年5月1日