[LeetCode] Binary Search Tree Iterator
來源:程序員人生 發布時間:2015-01-16 08:52:29 閱讀次數:2681次
Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.
Calling next()
will return the next smallest number in the BST.
Note: next()
and hasNext()
should
run in average O(1) time and uses O(h) memory, where h is the height of the tree.
這是1道很經典的題目,考的非遞歸的中序遍歷。其實這道題等價于寫1個2叉樹中序遍歷的迭代器。需要內置1個棧,1開始先存儲到最左葉子節點的路徑。在遍歷的進程中,只要當前節點存在右孩子,則進入右孩子,存除從此處開始到當前子樹里最左葉子節點的路徑。
public class BSTIterator {
Stack<TreeNode> stack;
public BSTIterator(TreeNode root) {
stack = new Stack<TreeNode>();
while (root != null) {
stack.push(root);
root = root.left;
}
}
/** @return whether we have a next smallest number */
public boolean hasNext() {
return !stack.isEmpty();
}
/** @return the next smallest number */
public int next() {
TreeNode node = stack.pop();
int ret = node.val;
if (node.right != null) {
node = node.right;
while (node != null) {
stack.push(node);
node = node.left;
}
}
return ret;
}
}
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