前序遍历和中序遍历树构造二叉树

• public class TreeNode {

• public int val;
  

• public TreeNode left, right;
  

• public TreeNode(int val) {
  

•     this.val = val;
  

•     this.left = this.right = null;
  

• }
  

• }
  /
  public class Solution {
  /*
  *@param preorder : A list of integers that preorder traversal of a tree
  *@param inorder : A list of integers that inorder traversal of a tree
  *@return : Root of a tree
  /
  public TreeNode buildTree(int[] preorder, int[] inorder) {
  // write your code here
  vector pre_l,pre_r,in_l,in_r;//定义这四个变量来存储左右子树的前序和中序序列
  TreeNode root=NULL;
  int i=0;
  int index=0;
  if(!preorder.empty()||!inorder.empty())//如果序列中不为空，继续构建
  {
  root=new TreeNode(preorder[0]);

   for(i=0;i<inorder.size();i++)
   {
       if(preorder[0]==inorder[i])
       {
           index=i;//找出分割中序序列的分割点，左边为左子树的，右边为有子树的
         break;
       }
  
   }
   for(i=0;i<index;i++)//重新找出前面部分前中序列
   {
       pre_l.push_back(preorder[i+1]);
       in_l.push_back(inorder[i]);
   }
   for(i=index+1;i<inorder.size();i++)//找出后面部分的前中序列
   {
       pre_r.push_back(preorder[i]);
       in_r.push_back(inorder[i]);
   }
   //依次构建分割的部分，缩小区间，直至构建完
   root->left=buildTree(pre_l,in_l);
   root->right=buildTree(pre_r,in_r);
   }
   return root;
  

  }
  };