leetCode——从中序与后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/15/

/**
 * 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& inorder, vector& postorder) {
        if(inorder.size()==0) return NULL;
        if(postorder.size()==0) return NULL;
        if(inorder.size() != postorder.size()) return NULL;
        return creatTree(inorder,0,inorder.size()-1, postorder,0,postorder.size()-1);
    }
    TreeNode* creatTree(vector& inorder, int in_start, int in_end, 
                   vector& postorder,int post_start,int post_end){
        if(post_start > post_end) return NULL;
        int val = postorder[post_end];
        TreeNode* root = new TreeNode(val);
        int i = in_start;
        for(;i <= in_end; i++) if(inorder[i] == val) break;
        root->left = creatTree(inorder, in_start, i-1, 
                            postorder, post_start, post_end + i - in_end - 1);
        root->right = creatTree(inorder, i+1, in_end, postorder, post_end+i-in_end,post_end-1);
        return root;
    }
    
};