Create ConstructBinaryTreefromPreorderandInorderTraversal.cpp

C++/DataStructure/Tree/constructBinaryTreefromPreorderandInorderTraversal.cpp

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+    /*
+        Author : Yash Gosaliya
+        Date : 11/10/2021
+        Description : C++ program to construct tree using inorder and preorder traversals
+    */
+
+    //Link : https://www.geeksforgeeks.org/construct-tree-from-given-inorder-and-preorder-traversal/
+
+    //Time Complexity : O(N)
+    //Space Complexity : O(N)
+
+    #include <bits/stdc++.h>
+
+    using namespace std;
+
+    /* A binary tree node has data, pointer to left child
+    and a pointer to right child */
+    struct Node {
+      char data;
+      struct Node * left;
+      struct Node * right;
+    };
+
+    struct Node * newNode(char data) {
+      struct Node * node = new Node;
+      node -> data = data;
+      node -> left = node -> right = NULL;
+      return (node);
+    }
+
+    struct Node * buildTree(char in [], char pre[], int inStrt,
+      int inEnd, unordered_map < char, int > & mp) {
+      static int preIndex = 0;
+
+      if (inStrt > inEnd)
+        return NULL;
+
+      /* Pick current node from Preorder traversal using preIndex and increment preIndex */
+      char curr = pre[preIndex++];
+      struct Node * tNode = newNode(curr);
+
+      /* If this node has no children then return */
+      if (inStrt == inEnd)
+        return tNode;
+
+      /* Else find the index of this node in Inorder traversal */
+      int inIndex = mp[curr];
+
+      /* Using index in Inorder traversal, construct left and right subtress */
+      tNode -> left = buildTree( in , pre, inStrt, inIndex - 1, mp);
+      tNode -> right = buildTree( in , pre, inIndex + 1, inEnd, mp);
+
+      return tNode;
+    }
+
+    // This function mainly creates an unordered_map, then
+    // calls buildTree()
+    struct Node * buldTreeWrap(char in [], char pre[], int len) {
+      // Store indexes of all items so that we
+      // we can quickly find later
+      unordered_map < char, int > mp;
+      for (int i = 0; i < len; i++)
+        mp[ in [i]] = i;
+
+      return buildTree( in , pre, 0, len - 1, mp);
+    }
+
+    /* This function is here just to test buildTree() */
+    void printInorder(struct Node * node) {
+      if (node == NULL)
+        return;
+      printInorder(node -> left);
+      printf("%c ", node -> data);
+      printInorder(node -> right);
+    }
+
+    /* Driver program to test above functions */
+    int main() {
+      char in [] = {
+        'D',
+        'B',
+        'E',
+        'A',
+        'F',
+        'C'
+      };
+      char pre[] = {
+        'A',
+        'B',
+        'D',
+        'E',
+        'C',
+        'F'
+      };
+      int len = sizeof( in ) / sizeof( in [0]);
+
+      struct Node * root = buldTreeWrap( in , pre, len);
+
+      /* Let us test the built tree by printing
+      Inorder traversal */
+      printf("Inorder traversal of the constructed tree is \n");
+      printInorder(root);
+    }
+
+    /* Inorder traversal of the constructed tree is 
+    D B E A F C */