题解 | #从中序与后序遍历序列构造二叉树#

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
private:
    unordered_map<int, int> rec;
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param inorder int整型vector 中序遍历序列
     * @param postorder int整型vector 后序遍历序列
     * @return TreeNode类
     */
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        // write code here
        for(int i=0; i<inorder.size(); i++){
            rec[inorder[i]] = i;
        }
        
        TreeNode* root = bulid(inorder, postorder, 0, inorder.size()-1, 
                               0, postorder.size()-1);
        return root;
    }
    
    TreeNode* bulid(vector<int>& inorder, vector<int>& postorder, int is, int ie,
                   int ps, int pe){
        if(is > ie){
            return nullptr;
        }
        
        TreeNode* root = new TreeNode(0);
        root->val = postorder[pe];
        
        int temp = rec[root->val];
        
        root->left = bulid(inorder, postorder, is, temp-1, ps, ps+temp-1-is);
        root->right = bulid(inorder, postorder, temp+1, ie, ps+temp-is, pe-1);
        
        return root;
    }
    
};