LeetCode-in-Java
1028. Recover a Tree From Preorder Traversal
Hard
We run a preorder depth-first search (DFS) on the root of a binary tree.
At each node in this traversal, we output D dashes (where D is the depth of this node), then we output the value of this node. If the depth of a node is D, the depth of its immediate child is D + 1. The depth of the root node is 0.
If a node has only one child, that child is guaranteed to be the left child.
Given the output traversal of this traversal, recover the tree and return its root.
Example 1:
Input: traversal = “1-2–3–4-5–6–7”
Output: [1,2,5,3,4,6,7]
Example 2:
Input: traversal = “1-2–3—4-5–6—7”
Output: [1,2,5,3,null,6,null,4,null,7]
Example 3:
Input: traversal = “1-401–349—90–88”
Output: [1,401,null,349,88,90]
Constraints:
- The number of nodes in the original tree is in the range
[1, 1000]. 1 <= Node.val <= 109
Solution
import com_github_leetcode.TreeNode;
/*
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
public class Solution {
private int ptr = 0;
public TreeNode recoverFromPreorder(String traversal) {
return find(traversal, 0);
}
private TreeNode find(String traversal, int level) {
if (ptr == traversal.length()) {
return null;
}
int i = ptr;
int count = 0;
while (traversal.charAt(i) == '-') {
count++;
i++;
}
if (count == level) {
int start = i;
while (i < traversal.length() && traversal.charAt(i) != '-') {
i++;
}
int val = Integer.parseInt(traversal.substring(start, i));
ptr = i;
TreeNode root = new TreeNode(val);
root.left = find(traversal, level + 1);
root.right = find(traversal, level + 1);
return root;
} else {
return null;
}
}
}