Leetcode 1028. Recover a Tree From Preorder Traversal

Input: traversal = "1-2--3--4-5--6--7"
Output: [1,2,5,3,4,6,7]
Input: traversal = "1-2--3---4-5--6---7"
Output: [1,2,5,3,null,6,null,4,null,7]
Input: traversal = "1-401--349---90--88"
Output: [1,401,null,349,88,90]
  • The number of nodes in the original tree is in the range [1, 1000].
  • 1 <= Node.val <= 109
  1. we will see dashes and value, so we verify it’s level and value while scan through the string
  2. because it’s in-order traversal, if the stack.size bigger than level, will pop out from the stack
  3. if the stack.peek().left is null, will add it into stack.peek().left; else add it into stack.peek().right
  4. to return the result, which is the root. We will need to pop all the other TreeNode in stack, until the last one -> the root
  5. Trick: use three variable in two for loops. Track index, level and value at the same time. Make sure index is not larger than string.length, to avoid overflow.

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