LeetCode 106. 从中序与后序遍历序列构造二叉树

/**
 * 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:
    unordered_map<int,int>pos;
    TreeNode* dfs(vector<int>&in, vector<int>&post, int il, int ir, int postl, int postr){
        if(postl>postr)return NULL;
        int k=pos[post[postr]]-il;
        TreeNode *root=new TreeNode(post[postr]);
        root->left=dfs(in,post,il,il+k-1,postl,postl+k-1);
        root->right=dfs(in,post,il+k+1,ir,postl+k,postr-1);
        return root;
    }
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        int n=inorder.size();
        for(int i=0;i<n;i++)pos[inorder[i]]=i;
        return dfs(inorder,postorder,0,n-1,0,n-1);
    }
};