[LeetCode] 501. Find Mode in Binary Search Tree
Find Mode in Binary Search Tree
Given a binary search tree (BST) with duplicates, find all the mode(s) (the most frequently occurred element) in the given BST.
Assume a BST is defined as follows:
The left subtree of a node contains only nodes with keys less than or equal to the node’s key.
The right subtree of a node contains only nodes with keys greater than or equal to the node’s key.
Both the left and right subtrees must also be binary search trees.
For example:
Given BST [1,null,2,2],
return [2].
Note: If a tree has more than one mode, you can return them in any order.
Follow up: Could you do that without using any extra space? (Assume that the implicit stack space incurred due to recursion does not count).
解析
找到二叉樹中的眾數。
解法1:inorder(遞迴)
遞迴中序遍歷二叉樹,建立一個雜湊表,儲存一個最大個數。
class Solution { public: vector<int> findMode(TreeNode* root) { vector<int> res; int mx = 0; unordered_map<int, int> m; inorder(root, m, mx); for (auto a : m) { if (a.second == mx) { res.push_back(a.first); } } return res; } void inorder(TreeNode* root, unordered_map<int, int>& m, int& mx){ if(!root) return; inorder(root->left, m, mx); m[root->val] ++; mx = max(mx, m[root->val]); inorder(root->right, m, mx); } };
解法2:inorder(迭代)
迭代中序遍歷二叉樹,建立一個雜湊表,儲存一個最大個數。
class Solution { public: vector<int> findMode(TreeNode* root) { if(!root) return {}; vector<int> res; int mx = 0; unordered_map<int, int> m; stack<TreeNode*> s; TreeNode* p = root; while(!s.empty() || p){ while(p){ s.push(p); p = p->left; } p = s.top(); s.pop(); m[p->val] ++; mx = max(mx, m[p->val]); p = p->right; } for (auto a : m) { if (a.second == mx) { res.push_back(a.first); } } return res; } };
解法3:inorder(遞迴不使用額外空間)
利用二叉樹的中序遍歷有序的特性,對二叉樹遞迴中序遍歷,設定一個pre節點,一個count記錄當前節點次數,當pre不為空時,不是第一個節點,與前一個節點值比較,如果相等則count+1,否則count=1;
當count大於最大次數mx時,清空res,加入當前節點,並將mx=count;pre更新為當前節點p。
class Solution {
public:
vector<int> findMode(TreeNode* root) {
vector<int> res;
int mx = 0;
int count = 1;
TreeNode* pre = NULL;
inorder(root, pre, count, mx, res);
return res;
}
void inorder(TreeNode* p, TreeNode*& pre, int& count,int& mx, vector<int>& res){
if(!p) return;
inorder(p->left, pre, count,mx,res);
if(pre){
count = (pre->val == p->val) ? count+1:1;
}
if(count>=mx){
if(count>mx) res.clear();
res.push_back(p->val);
mx = count;
}
pre = p;
inorder(p->right, pre, count,mx,res);
}
};
解法4:inorder(迭代不使用額外空間)
迭代中序遍歷,類似於解法3
class Solution {
public:
vector<int> findMode(TreeNode* root) {
if(!root) return {};
vector<int> res;
int mx = 0;
int count =1;
stack<TreeNode*> s;
TreeNode* p = root;
TreeNode* pre = NULL;
while(!s.empty() || p){
while(p){
s.push(p);
p = p->left;
}
p = s.top();s.pop();
if(pre){
count = (pre->val == p->val) ? count+1:1;
}
if(count>=mx){
if(count>mx) res.clear();
res.push_back(p->val);
mx = count;
}
pre = p;
p = p->right;
}
return res;
}
};