105. Construct Binary Tree from Preorder and Inorder Traversal.cpp

add the solution of leetcode question number 105

Author: PralayeshMukherjee

105. Construct Binary Tree from Preorder and Inorder Traversal.cpp

@@ -0,0 +1,32 @@
+/**
+ * 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> mp;
+    TreeNode* build(int il,int ir,int pl,int pr,vector<int> &preorder,vector<int> &inorder){
+        if(il>ir) return NULL;
+        TreeNode* cur_node = new TreeNode(preorder[pl]);
+        if(il==ir) return cur_node;
+        int inorder_ind = mp[preorder[pl]];
+        int m = pl+inorder_ind-il;
+        cur_node->left = build(il,inorder_ind-1,pl+1,m,preorder,inorder);
+        cur_node->right = build(inorder_ind+1,ir,m+1,pr,preorder,inorder);
+        return cur_node;
+    }
+    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
+        int n = inorder.size();
+        for(int i=0;i<n;i++){
+            mp[inorder[i]] = i;
+        }
+        return build(0,n-1,0,n-1,preorder,inorder);
+    }
+};