Use HashMap so we don't need to "find"

Author: keineahnung2345

106. Construct Binary Tree from Inorder and Postorder Traversal.cpp

@@ -37,3 +37,50 @@ class Solution {
         return node;
     }
 };
+
+//Use HashMap so we don't need to "find"
+//https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/discuss/34782/My-recursive-Java-code-with-O(n)-time-and-O(n)-space
+//Runtime: 24 ms, faster than 74.28% of C++ online submissions for Construct Binary Tree from Inorder and Postorder Traversal.
+//Memory Usage: 24 MB, less than 36.01% of C++ online submissions for Construct Binary Tree from Inorder and Postorder Traversal.
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    unordered_map<int, int> val2idx;
+    
+    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder, int is, int ie, int ps, int pe) {
+        if(is > ie || ps > pe) return nullptr;
+        
+        TreeNode* node = new TreeNode(postorder[pe]);
+        
+        int iroot = val2idx[postorder[pe]];
+        // cout << "inorder's root at: " << iroot << endl;
+        int leftSize = iroot-is;
+        
+        node->left = buildTree(inorder, postorder, is, iroot-1, ps, ps+leftSize-1);
+        node->right = buildTree(inorder, postorder, iroot+1, ie, ps+leftSize, pe-1);
+        
+        return node;
+    };
+    
+    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
+        int n = inorder.size();
+        
+        if(n == 0) return nullptr;
+        
+        for(int i = 0; i < n; ++i){
+            val2idx[inorder[i]] = i;
+        }
+        
+        return buildTree(inorder, postorder, 0, n-1, 0, n-1);
+    }
+};