Construct Binary Tree from Preorder and Inorder Traversal LeetCode Solution

Problem – Construct Binary Tree from Preorder and Inorder Traversal LeetCode Solution

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.

Construct Binary Tree from Preorder and Inorder Traversal LeetCode Solution in Java

public TreeNode buildTree(int[] preorder, int[] inorder) {
    return helper(0, 0, inorder.length - 1, preorder, inorder);
}

public TreeNode helper(int preStart, int inStart, int inEnd, int[] preorder, int[] inorder) {
    if (preStart > preorder.length - 1 || inStart > inEnd) {
        return null;
    }
    TreeNode root = new TreeNode(preorder[preStart]);
    int inIndex = 0; // Index of current root in inorder
    for (int i = inStart; i <= inEnd; i++) {
        if (inorder[i] == root.val) {
            inIndex = i;
        }
    }
    root.left = helper(preStart + 1, inStart, inIndex - 1, preorder, inorder);
    root.right = helper(preStart + inIndex - inStart + 1, inIndex + 1, inEnd, preorder, inorder);
    return root;
}

Construct Binary Tree from Preorder and Inorder Traversal LeetCode Solution in Python

def buildTree(self, preorder, inorder):
    if inorder:
        ind = inorder.index(preorder.pop(0))
        root = TreeNode(inorder[ind])
        root.left = self.buildTree(preorder, inorder[0:ind])
        root.right = self.buildTree(preorder, inorder[ind+1:])
        return root

Construct Binary Tree from Preorder and Inorder Traversal LeetCode Solution in C++

class Solution {
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        int rootIdx = 0;
        return build(preorder, inorder, rootIdx, 0, inorder.size()-1);
    }
    
    TreeNode* build(vector<int>& preorder, vector<int>& inorder, int& rootIdx, int left, int right) {
        if (left > right) return NULL;
        int pivot = left;  // find the root from inorder
        while(inorder[pivot] != preorder[rootIdx]) pivot++;
        
        rootIdx++;
        TreeNode* newNode = new TreeNode(inorder[pivot]);
        newNode->left = build(preorder, inorder, rootIdx, left, pivot-1);
        newNode->right = build(preorder, inorder, rootIdx, pivot+1, right);
        return newNode;
    }
};

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