leetcode_middle_95_105. Construct Binary Tree from Preorder and Inorder Traversal

题意：

给一个二叉树的前序遍历和中序遍历结果，构造出这棵树。

分析：

分析同上一篇。

/** * 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(preorder.length==0 || inorder.length==0)              return null;          return helper(preorder, inorder, 0, 0, inorder.length-1);      }        private TreeNode helper(int[] preorder, int[] inorder, int rootindex, int start, int end){          if(start > end){              return null;          }          int rootval = preorder[rootindex];          TreeNode root = new TreeNode(rootval);          int index = findRootInInorder(rootval, inorder, start, end);          int leftsize = index - start;          root.left = helper(preorder, inorder, rootindex+1, start, index-1);          root.right = helper(preorder, inorder, rootindex+1+leftsize, index+1, end);          return root;      }            private int findRootInInorder(int rootval, int[] inorder, int start, int end){          for(int i=start; i<=end; i++){              if(inorder[i] == rootval){                  return i;              }          }          return -1;      }  }