Tree
Binary Search Tree
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
if (root->val > p->val && root->val > q->val) {
return lowestCommonAncestor(root->left, p, q);
} else if (root->val < p->val && root->val < q->val) {
return lowestCommonAncestor(root->right, p, q);
} else {
return root;
}
}
};TODO
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
int K;
public int kthSmallest(TreeNode root, int k) {
K = k;
return inorder(root);
}
private Integer inorder(TreeNode cur) {
if (cur == null)
return null;
Integer left = inorder(cur.left);
if (left == null) {
if (--K == 0)
return cur.val;
else
return inorder(cur.right);
}
return left;
}
}Binary Tree
class Solution {
public TreeNode invertTree(TreeNode root) {
if (root == null) {
return null;
}
invertTree(root.left);
invertTree(root.right);
TreeNode tmp = root.left;
root.left = root.right;
root.right = tmp;
return root;
}
}Double check how to implement post-order visiting using iterative way.
class Solution {
public:
bool isBalanced(TreeNode* root) {
return isBalancedIterative(root);
}
bool isBalancedRecursive(TreeNode* root) {
if (helper(root) == -1) {
return false;
}
return true;
}
int helper(TreeNode* node) {
if (!node) {
return 0;
}
int leftHeight = helper(node->left);
int rightHeight = helper(node->right);
if (abs(leftHeight - rightHeight) > 1) {
return -1;
} else if (leftHeight == -1 || rightHeight == -1) {
return -1;
} else {
return max(leftHeight, rightHeight) + 1;
}
}
bool isBalancedIterative(TreeNode* root) {
if (!root) return true;
std::stack<TreeNode*> stack;
std::unordered_map<TreeNode*, int> heights;
TreeNode* lastVisited = nullptr;
TreeNode* current = root;
while (!stack.empty() || current) {
// Reach the leftmost node
while (current) {
stack.push(current);
current = current->left;
}
current = stack.top();
// If the right child exists and is not yet visited, visit it
if (current->right && lastVisited != current->right) {
current = current->right;
} else {
stack.pop();
int leftHeight = heights.count(current->left) ? heights[current->left] : 0;
int rightHeight = heights.count(current->right) ? heights[current->right] : 0;
// Check if the current node is balanced
if (std::abs(leftHeight - rightHeight) > 1) {
return false;
}
// Store the height of the current node
heights[current] = 1 + std::max(leftHeight, rightHeight);
lastVisited = current;
current = nullptr;
}
}
return true;
}
};/**
* 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:
vector<vector<int>> levelOrder(TreeNode* root) {
vector<vector<int>> results;
if (!root) {
return results;
}
queue<TreeNode*> q;
q.push(root);
while(!q.empty()) {
int size = q.size();
vector<int> tmp;
for (int i = 0; i < size; i++) {
TreeNode* n = q.front();
tmp.push_back(n->val);
if (n->left) {
q.push(n->left);
}
if (n->right) {
q.push(n->right);
}
q.pop();
}
results.push_back(tmp);
}
return results;
}
};class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if (root == null) {
return null;
}
if (root == p || root == q) {
return root;
}
TreeNode l = lowestCommonAncestor(root.left, p, q);
TreeNode r = lowestCommonAncestor(root.right, p, q);
if (l != null && r != null) {
return root;
} else if (l != null) {
return l;
} else {
return r;
}
}
}/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Codec {
public:
// Encodes a tree to a single string.
string serialize(TreeNode* root) {
if (!root) {
return "x,";
}
return to_string(root->val) + "," + serialize(root->left) + serialize(root->right);
}
// Decodes your encoded data to tree.
TreeNode* deserialize(string data) {
queue<string> q;
string s;
for (int i = 0; i < data.size(); i++) {
if (data[i] == ',') {
q.push(s);
s = "";
continue;
}
s += data[i];
}
if (s.size() != 0) {
q.push(s);
}
return deserialize_helper(q);
}
TreeNode* deserialize_helper(queue<string> &q) {
string s = q.front();
q.pop();
if (s == "x") {
return NULL;
}
TreeNode* root = new TreeNode(stoi(s));
root->left = deserialize_helper(q);
root->right = deserialize_helper(q);
return root;
}
};
// Your Codec object will be instantiated and called as such:
// Codec ser, deser;
// TreeNode* ans = deser.deserialize(ser.serialize(root));class Solution {
private:
int result;
public:
int diameterOfBinaryTree(TreeNode* root) {
iterative(root);
return result;
}
void iterative(TreeNode* node) {
stack<TreeNode*> s;
TreeNode* current = node;
TreeNode* lastVisited = nullptr;
unordered_map<TreeNode*, int> heights;
while (!s.empty() || current) {
while (current) {
s.push(current);
current = current->left;
}
current = s.top();
if (current->right && current->right != lastVisited) {
current = current->right;
} else {
s.pop();
int left = heights.count(current->left) ? heights[current->left] : 0;
int right = heights.count(current->right) ? heights[current->right] : 0;
result = max(result, left + right); // post-order
heights[current] = max(left, right) + 1;
lastVisited = current;
current = nullptr;
}
}
}
int recursive(TreeNode* node) {
if (!node) {
return 0;
}
int left = recursive(node->left);
int right = recursive(node->right);
result = max(result, left + right); // post-order
return max(left, right) + 1;
}
};class Solution {
public List<Integer> rightSideView(TreeNode root) {
List<Integer> result = new ArrayList<>();
Map<Integer, Boolean> marked = new HashMap<>();
rightSideRecursive(result, root, marked, 0);
return result;
}
public void rightSideRecursive(List<Integer> result, TreeNode cur, Map<Integer, Boolean> marked, int level) {
if (cur == null) {
return;
}
if (!marked.getOrDefault(level, false)) {
marked.put(level, true);
result.add(cur.val);
}
rightSideRecursive(result, cur.right, marked, level+1);
rightSideRecursive(result, cur.left, marked, level+1);
}
}class Solution {
public:
int maxDepth(TreeNode* root) {
return helper(root);
}
int helper(TreeNode *node) {
if (!node) {
return 0;
}
return max(helper(node->left), helper(node->right)) + 1;
}
};class Solution {
public int maxDepth(TreeNode root) {
Stack<TreeNode> s = new Stack<>();
TreeNode current = root;
TreeNode last = null;
Map<TreeNode, Integer> h = new HashMap<>();
while (!s.empty() || current != null) {
while (current != null) {
s.push(current);
current = current.left;
}
current = s.peek();
if (current.right != null && last != current.right) {
current = current.right;
} else {
s.pop();
// post-order visit
int left = h.containsKey(current.left) ? h.get(current.left) : 0;
int right = h.containsKey(current.right) ? h.get(current.right) : 0;
h.put(current, (int)(Math.max(left, right) + 1));
last = current;
current = null;
}
}
return h.get(root);
}
}TODO
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