2022-07-14 Daily-Challenge

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Today I have done leetcode's July LeetCoding Challenge with cpp.

July LeetCoding Challenge 14

Description

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:

img

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.

Solution

class Solution {
  TreeNode *build(
    vector<int>& preorder,
    int preBegin,
    int preEnd,
    vector<int>& inorder,
    int inBegin,
    int inEnd
  ) {
    if(preBegin > preEnd) return nullptr;
    int rootIdx = -1;
    for(int i = inBegin; i <= inEnd; ++i) {
      if(inorder[i] == preorder[preBegin]) {
        rootIdx = i;
        break;
      }
    }
    int offset = rootIdx - inBegin;
    TreeNode *root = new TreeNode(preorder[preBegin]);
    root->left = build(preorder, preBegin + 1, preBegin + offset, inorder, inBegin, inBegin + offset - 1);
    root->right = build(preorder, preBegin + offset + 1, preEnd, inorder, inBegin + offset + 1, inEnd);
    return root;
  }
public:
  TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
    int len = preorder.size();
    return build(preorder, 0, len - 1, inorder, 0, len - 1);
  }
};

// Accepted
// 203/203 cases passed (44 ms)
// Your runtime beats 40.32 % of cpp submissions
// Your memory usage beats 79.05 % of cpp submissions (25.9 MB)
Algorithm