Update and rename buildTreeWithPostOrder.cpp to construct-binary-tree-from-inorder-and-postorder-traversal.cpp

Author: kamyu104

C++/buildTreeWithPostOrder.cpp

@@ -0,0 +1,43 @@
+// Time:  O(n)
+// Space: O(n)
+
+/**
+ * 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<int>& inorder, vector<int>& postorder) {
+        unordered_map<int, size_t> in_entry_idx_map;
+        for (size_t i = 0; i < inorder.size(); ++i) {
+            in_entry_idx_map.emplace(inorder[i], i);
+        }
+        return ReconstructPostInOrdersHelper(postorder, 0, postorder.size(), inorder, 0, inorder.size(),
+                                             in_entry_idx_map);
+    }
+
+    TreeNode * ReconstructPostInOrdersHelper(const vector<int>& postorder, size_t post_s, size_t post_e,
+                                             const vector<int>& inorder, size_t in_s, size_t in_e,
+                                             const unordered_map<int, size_t>& in_entry_idx_map) {
+        if (post_s == post_e || in_s == in_e) {
+            return nullptr;
+        }
+        
+        auto idx = in_entry_idx_map.at(postorder[post_e - 1]);
+        auto left_tree_size = idx - in_s;
+
+        TreeNode *node = new TreeNode(postorder[post_e - 1]);
+        // Recursively builds the left subtree.
+        node->left =ReconstructPostInOrdersHelper(postorder, post_s, post_s + left_tree_size,
+                                                  inorder, in_s, idx, in_entry_idx_map);
+        // Recursively builds the right subtree.
+        node->right = ReconstructPostInOrdersHelper(postorder, post_s + left_tree_size, post_e - 1,
+                                                    inorder, idx + 1, in_e, in_entry_idx_map);
+        return node;
+    }
+};