Construct Binary Tree from Preorder and Inorder Traversal

Author: VishalSingh-07

Medium/Construct_Binary_Tree_from_Preorder_and_Inorder_Traversal.cpp

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+/*
+105. Construct Binary Tree from Preorder and Inorder Traversal
+
+Given two integer arrays preorder and inorder where preorder is the preorder traversal of a binary tree and inorder is the inorder traversal of the same tree, construct and return the binary tree.
+
+ 
+
+Example 1:
+
+
+Input: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
+Output: [3,9,20,null,null,15,7]
+
+
+Example 2:
+
+Input: preorder = [-1], inorder = [-1]
+Output: [-1]
+ 
+
+Constraints:
+- 1 <= preorder.length <= 3000
+- inorder.length == preorder.length
+- -3000 <= preorder[i], inorder[i] <= 3000
+- preorder and inorder consist of unique values.
+- Each value of inorder also appears in preorder.
+- preorder is guaranteed to be the preorder traversal of the tree.
+- inorder is guaranteed to be the inorder traversal of the tree.
+*/
+
+/**
+ * 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) {}
+ * };
+ */
+
+#include <bits/stdc++.h>
+using namespace std;
+
+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) {}
+};
+
+/*
+Optimized Approach
+Time complexity -> O(n) and Space -> O(n)
+where:
+- n: number of nodes in a binary tree
+- inStart: inorderStart
+- inEnd: inorderEnd
+- preStart: postorderStart
+- preEnd: postorderEnd
+*/
+class Solution {
+ TreeNode* constructBinaryTree(vector<int> &inorder, int inStart,int inEnd,vector<int> &preorder, int preStart, int preEnd, map<int,int> &mpp){
+ if(preStart>preEnd || inStart>inEnd){
+ return nullptr;
+ }
+ TreeNode *root=new TreeNode(preorder[preStart]);
+ int inRoot=mpp[preorder[preStart]];
+ int numsLeft=inRoot-inStart;
+ root->left=constructBinaryTree(inorder,inStart,inRoot-1,preorder,preStart+1,preStart+numsLeft,mpp);
+ root->right=constructBinaryTree(inorder,inRoot+1,inEnd,preorder,preStart+numsLeft+1,preEnd,mpp);
+ return root;
+ }
+public:
+ TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
+ if(inorder.size()!=preorder.size()){
+ return nullptr;
+ }
+ map<int,int> mpp;
+ for(int i=0;i<inorder.size();i++){
+ mpp[inorder[i]]=i;
+ }
+ return constructBinaryTree(inorder,0,inorder.size()-1, preorder,0,preorder.size()-1,mpp);
+ }
+};
+
+/*
+1. Question link -- https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/description/
+
+2. Solution link -- https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/solutions/5116112/optimized-approach-with-detailed-explanation-best-c-solution-striver-solution
+
+3. Video link -- https://youtu.be/aZNaLrVebKQ
+*/