156 Binary Tree Upside Down
156. Binary Tree Upside Down
1. Question
Given a binary tree where all the right nodes are either leaf nodes with a sibling (a left node that shares the same parent node) or empty, flip it upside down and turn it into a tree where the original right nodes turned into left leaf nodes. Return the new root.
For example:
Given a binary tree{1,2,3,4,5}
,
1
/ \
2 3
/ \
4 5
return the root of the binary tree[4,5,2,#,#,3,1]
.
4
/ \
5 2
/ \
3 1
2. Implementation
(1) Recursion
class Solution {
public TreeNode upsideDownBinaryTree(TreeNode root) {
if (root == null || root.left == null) {
return root;
}
TreeNode newRoot = upsideDownBinaryTree(root.left);
root.left.left = root.right;
root.left.right = root;
root.left = null;
root.right = null;
return newRoot;
}
}
(2) Iteration
class Solution {
public TreeNode upsideDownBinaryTree(TreeNode root) {
TreeNode curNode = root, preNode = null, nextNode = null, tempNode = null;
while (curNode != null) {
nextNode = curNode.left;
curNode.left = tempNode;
tempNode = curNode.right;
curNode.right = preNode;
preNode = curNode;
curNode = nextNode;
}
return preNode;
}
}
3. Time & Space Complexity
Recursion: 时间复杂度O(h), 空间复杂度O(h)
Iteration: 时间复杂度O(h), 空间复杂度O(1)
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