# exact O(h) time

• ``````/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
// two cases
// (1) right tree is null, just let leftsubtree replace current node
// (2) right tree is not null,
// find right tree leftmost node, put current node left tree to this node left tree
// use current node right tree replace current node
TreeNode preNode = null;
public TreeNode deleteNode(TreeNode root, int key) {
TreeNode node = findNode(root, key);
if (node == null) return root;

// node.right == null case
if (node.right == null) {
if (preNode == null) return node.left;
if (preNode.val < key) {
preNode.right = node.left;
} else {
preNode.left = node.left;
}
return root;
}

// node has right
TreeNode tmp = node.right;
while (tmp.left != null) tmp = tmp.left;
tmp.left = node.left;
if (preNode == null) return node.right;
if (preNode.val < key) {
preNode.right = node.right;
} else {
preNode.left = node.right;
}
return root;
}
private TreeNode findNode(TreeNode root, int key) {
if (root == null) return null;
if (root.val == key) return root;
if (root.val < key) {
preNode = root;
return findNode(root.right, key);
}
preNode = root;
return findNode(root.left, key);
}
}``````

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