Added 106. Construct Binary Tree from Inorder and Postorder Traversal

Author: tarunsinghdev

leetcode/cpp/Tree/106.cpp

@@ -0,0 +1,25 @@
+class Solution {
+private : unordered_map<int, int> mp;
+public:
+    TreeNode* buildTree_util(vector<int>& inorder, int  i1, int i2, vector<int>& postorder, int p1, int p2){
+        if(i1 > i2 || p1 > p2)
+            return NULL; 
+        TreeNode* root = new TreeNode(postorder[p2]);
+        int diff = mp[postorder[p2]] - i1;
+        root->left = buildTree_util(inorder, i1 , i1+diff-1, postorder, p1, p1+diff-1);
+        root->right = buildTree_util(inorder, i1+diff+1, i2, postorder, p1+diff, p2-1);
+        
+        return root;
+        
+        
+    }
+    
+    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
+        int n = inorder.size();
+        if(n == 0) return NULL;
+        for(int i = 0; i < n; i++) 
+            mp[inorder[i]] = i;  //(inorder, index)
+        
+        return buildTree_util(inorder, 0, n-1, postorder, 0, n-1);
+    }
+};