LeetCode-230. Kth Smallest Element in a BST (JAVA)二叉树第k小的数字

230. Kth Smallest Element in a BST

Given a binary search tree, write a function

kthSmallest

to find the kth smallest element in it.

Note:

You may assume k is always valid, 1 ? k ? BST's total elements.

BST中第k小的数字，中序遍历，没啥好说的。

private TreeNode temp;
private int counter;

public int kthSmallest(TreeNode root, int k) {
inorder(root, k);
return temp.val;
}

public void inorder(TreeNode node, int k) {
// perform in order tree traversal
if (node == null)
return; // base case
inorder(node.left, k);
if (++counter == k) {
temp = node;
return;
}
inorder(node.right, k);
}
迭代：

// k输入有效，中序遍历
public int kthSmallest(TreeNode root, int k) {
Stack<TreeNode> stk = new Stack<>();
while (root != null || !stk.isEmpty()) {
if (root != null) {
stk.push(root);
root = root.left;
} else {
root = stk.pop();
if (--k == 0)
return root.val;
root = root.right;
}
}
// 此题k输入合法，不会到达这里
return Integer.MIN_VALUE;

}