814 Binary Tree Pruning
814. Binary Tree Pruning
1. Question
We are given the head noderoot
of a binary tree, where additionally every node's value is either a 0 or a 1.
Return the same tree where every subtree (of the given tree) not containing a 1 has been removed.
(Recall that the subtree of a node X is X, plus every node that is a descendant of X.)
Example 1:
Input:
1
\
0
/ \
0 1
Output:
1
\
0
\
1
Explanation:
Only the red nodes satisfy the property "every subtree not containing a 1".
The diagram on the right represents the answer.
2. Implementation
(1) DFS
思路: post order traversal, 先处理左子树和右子树,然后如果当前node是leaf且它的值为0的话,把该node删除
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public TreeNode pruneTree(TreeNode root) {
if (root == null) {
return null;
}
root.left = pruneTree(root.left);
root.right = pruneTree(root.right);
if (root.left == null && root.right == null && root.val == 0) {
return null;
}
return root;
}
}
3. Time & Space Complexity
DFS: 时间复杂度O(n), 空间复杂度: O(h)
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