leetCode——从前序与中序遍历序列构造二叉树

题目：https://leetcode-cn.com/explore/learn/card/data-structure-binary-tree/4/conclusion/16/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* buildTree(vector& preorder, vector& inorder) {
         if(preorder.size()==0) return NULL;
         if(inorder.size()==0) return NULL;
         if(preorder.size() != inorder.size()) return NULL;
         return creatTree(preorder, 0 , preorder.size()-1,
                         inorder,0, inorder.size()-1);
    }
    TreeNode* creatTree(vector& preorder, int pre_start, int pre_end, 
                   vector& inorder,int in_start,int in_end){
        if(in_start > in_end) return NULL;
        int val = preorder[pre_start];
        TreeNode* root = new TreeNode(val);
        int i = in_start;
        for(;i <= in_end; i++) if(inorder[i] == val) break;
        root->left = creatTree(preorder, pre_start+1, pre_start+i-in_start,
                              inorder, in_start,i-1 );
        root->right = creatTree(preorder, pre_start+i-in_start+1,pre_end,
                               inorder, i+1,in_end);
        return root;
    }
};