Kth Smallest Element in a BST
Solution
Using DFS
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
int smallestK;
int count = 0;
int k = 0;
int kthSmallest(TreeNode* root, int k) {
this->k = k;
dfs(root);
return smallestK;
}
void dfs(TreeNode* root) {
if(root == NULL) return;
dfs(root->left);
count++;
if(count == k) {
smallestK = root->val;
return;
}
dfs(root->right);
}
};
Updating Size at the time of Building Tree
class Node {
public:
int val;
int size;
Node *left;
Node *right;
Node(int v) {
val = v;
size = 1;
left = nullptr;
right = nullptr;
}
};
class BinarySearchTree {
public:
Node* root = nullptr;
int size = 0;
BinarySearchTree() {}
void addNum(int val) {
root = addNode(root, val);
size++;
// printf("Added: %d\n", root->size);
}
int findKthNum(int k) {
Node* n = findKthNode(root, k);
if (n != nullptr) {
return n->val;
}
return -1;
}
void printTree(Node* root, int indent) {
if (root == nullptr) {
return;
}
for (int i = 0; i < indent; i++) {
printf(" ");
}
cout << root;
printf(" %d (Size: %d)\n", root->val, root->size);
// cout << root << " " << "Size"
printTree(root->left, indent+1);
printTree(root->right, indent+1);
}
Node* addNode(Node* root, int val) {
if (root == nullptr) {
return new Node(val);
}
root->size++;
// printf("Inserting %d\t", val);
// printf("at node: %d Size: %d\n", root->val, root->size);
if (root->val >= val) {
root->left = addNode(root->left, val);
return root;
}
root->right = addNode(root->right, val);
return root;
}
Node* findKthNode(Node* root, int k) {
if (root == nullptr) { // assume that we never reach here
return nullptr;
}
// printf("Finding %d at ", k);
// cout << root << "\n";
int leftSize = 0;
if (root->left != nullptr) {
leftSize = root->left->size;
}
if (k <= leftSize) {
return findKthNode(root->left, k);
} else if (k == leftSize + 1) {
return root;
} else {
return findKthNode(root->right, k - 1 - leftSize);
}
}
};
/**
* Your MedianFinder object will be instantiated and called as such:
* MedianFinder* obj = new MedianFinder();
* obj->addNum(num);
* double param_2 = obj->findMedian();
*/
Also check: Find Median from Data Stream
