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Closest Binary Search Tree Value II in C++
Suppose we have a binary search tree and a target value; we have to find k values in that BST that are closest to the target. We have to keep in mind that the target value is a floating-point number. We can assume k is always valid, and k ≤ total nodes.
So, if the input is like
target = 3.714286, and k = 2, then the output will be [4,3]
To solve this, we will follow these steps −
Define a function pushSmaller(), this will take node,stack st, and target,
while node is not present, do −
if val of node < target, then −
insert node into st
node := right of node
Otherwise
node := left of node
Define a function pushLarger(), this will take node, stack st, target,
while node is empty, do −
if val of node >= target, then −
insert node into st
node := left of node
Otherwise
node := right of node
From the main method do the following −
Define an array ret
Define one stack smaller
Define one stack larger
pushLarger(root, larger, target)
pushSmaller(root, smaller, target)
while k is non-zero, decrease k in each step, do −
if smaller is not empty and (larger is empty or |target - value of top element of smaller| < |target - top element of larger|)
curr = top element of smaller
delete element from smaller
insert val of curr at the end of ret
pushSmaller(left of curr, smaller, target)
Otherwise
curr = top element of larger
delete element from larger
insert val of curr at the end of ret
pushSmaller(right of curr, larger, target)
return ret
Example
Let us see the following implementation to get a better understanding −
#include <bits/stdc++.h>
using namespace std;
void print_vector(vector<auto> v){
cout << "[";
for(int i = 0; i<v.size(); i++){
cout << v[i] << ", ";
}
cout << "]"<<endl;
}
class TreeNode{
public:
int val;
TreeNode *left, *right;
TreeNode(int data){
val = data;
left = NULL;
right = NULL;
}
};
void insert(TreeNode **root, int val){
queue<TreeNode*> q;
q.push(*root);
while(q.size()){
TreeNode *temp = q.front();
q.pop();
if(!temp->left){
if(val != NULL)
temp->left = new TreeNode(val);
else
temp->left = new TreeNode(0);
return;
}
else{
q.push(temp->left);
}
if(!temp->right){
if(val != NULL)
temp->right = new TreeNode(val);
else
temp->right = new TreeNode(0);
return;
}
else{
q.push(temp->right);
}
}
}
TreeNode *make_tree(vector<int> v){
TreeNode *root = new TreeNode(v[0]);
for(int i = 1; i<v.size(); i++){
insert(&root, v[i]);
}
return root;
}
class Solution {
public:
vector<int> closestKValues(TreeNode* root, double target, int k) {
vector<int> ret;
stack<TreeNode*> smaller;
stack<TreeNode*> larger;
pushLarger(root, larger, target);
pushSmaller(root, smaller, target);
while (k--) {
if (!smaller.empty() && (larger.empty() || (abs(target - smaller.top()->val) < abs(target - larger.top()->val)))) {
TreeNode* curr = smaller.top();
smaller.pop();
ret.push_back(curr->val);
pushSmaller(curr->left, smaller, target);
}
else {
TreeNode* curr = larger.top();
larger.pop();
ret.push_back(curr->val);
pushLarger(curr->right, larger, target);
}
}
return ret;
}
void pushSmaller(TreeNode* node, stack <TreeNode*>& st, double target){
while (node) {
if (node->val < target) {
st.push(node);
node = node->right;
}
else {
node = node->left;
}
}
}
void pushLarger(TreeNode* node, stack <TreeNode*>& st, double target){
while (node) {
if (node->val >= target) {
st.push(node);
node = node->left;
}
else
node = node->right;
}
}
};
main(){
Solution ob;
vector<int> v = {4,2,5,1,3};
TreeNode *root = make_tree(v);
print_vector(ob.closestKValues(root, 3.7142, 2));
}Input
{4,2,5,1,3}, 3.7142, 2Output
[4, 3, ]
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