2021-11-21 Daily-Challenge

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Posted modified:
In: DailyChallenge

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

November LeetCoding Challenge 21

Description

Construct Binary Tree from Inorder and Postorder Traversal

Given two integer arrays inorder and postorder where inorder is the inorder traversal of a binary tree and postorder is the postorder traversal of the same tree, construct and return the binary tree.

Example 1:

img

Input: inorder = [9,3,15,20,7], postorder = [9,15,7,20,3]
Output: [3,9,20,null,null,15,7]

Example 2:

Input: inorder = [-1], postorder = [-1]
Output: [-1]

Constraints:

  • 1 <= inorder.length <= 3000
  • postorder.length == inorder.length
  • -3000 <= inorder[i], postorder[i] <= 3000
  • inorder and postorder consist of unique values.
  • Each value of postorder also appears in inorder.
  • inorder is guaranteed to be the inorder traversal of the tree.
  • postorder is guaranteed to be the postorder traversal of the tree.

Solution

auto speedup = [](){
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  return 0;
}();
class Solution {
  TreeNode* buildTree(
    vector<int> &inorder, 
    int inBegin,
    int inEnd,
    vector<int> &postorder,
    int postBegin,
    int postEnd
  ) {
    if(inBegin > inEnd) return nullptr;
    TreeNode *root = new TreeNode(postorder[postEnd]);
    for(int i = inBegin; i <= inEnd; ++i) {
      if(inorder[i] == postorder[postEnd]) {
        int offset = i - inBegin;
        root->left = buildTree(inorder, inBegin, inBegin + offset - 1, postorder, postBegin, postBegin + offset - 1);
        root->right = buildTree(inorder, inBegin + offset + 1, inEnd, postorder, postBegin + offset, postEnd - 1);
        break;
      }
    }
    return root;
  }
public:
  TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
    int len = inorder.size();
    return buildTree(inorder, 0, len - 1, postorder, 0, len - 1);
  }
};

// Accepted
// 202/202 cases passed (32 ms)
// Your runtime beats 32.56 % of cpp submissions
// Your memory usage beats 62.12 % of cpp submissions (26.2 MB)
Algorithm