[crispy] Week 1 Solution explanation (#313)

* solve: contains-duplicate

* renmae to fit with contribution guide

* feat: add complexity

* solve: number of 1 bit

* docs: add tc and sc

* solve: top k frequent elements

* solve: kth smallest in bst

* solve: palindromic substring

* feat: add dp version of palindromic substring

Author: heozeop

contains-duplicate/heozeop.cpp

@@ -0,0 +1,20 @@
+// Time Complexity : O(n)
+// Spatial Complexity : O(n)
+
+#include <set>
+
+class Solution {
+public:
+    bool containsDuplicate(vector<int>& nums) {
+        set<int> count;
+
+        for(int num : nums) {
+            if (count.find(num) != count.end()) {
+                return true;
+            }
+            count.insert(num);
+        }
+
+        return false;
+    }
+};

kth-smallest-element-in-a-bst/heozeop.cpp

@@ -0,0 +1,46 @@
+// Time Complexity: O(nlogn)
+// Spatial Complexity O(n)
+
+#include <vector>
+#include <queue>
+/**
+ * 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 kthSmallest(TreeNode* root, int k) {
+        queue<TreeNode*> q;
+        q.push(root);
+
+        vector<int> numbers;
+        TreeNode *curNode, *nextNode;
+        while(!q.empty()) {
+            curNode = q.front();
+            q.pop();
+
+            numbers.push_back(curNode->val);
+
+            nextNode = curNode->left;
+            if(nextNode != nullptr) {
+                q.push(nextNode);
+            }
+
+            nextNode = curNode->right;
+            if(nextNode != nullptr) {
+                q.push(nextNode);
+            }
+        }
+
+        sort(numbers.begin(), numbers.end());
+        return numbers[k - 1];
+    }
+};
+

number-of-1-bits/heozeop.cpp

@@ -0,0 +1,19 @@
+// Time Complexity: O(log(n))
+// Spatial Complexity: O(1)
+
+class Solution {
+public:
+    int hammingWeight(int n) {
+        int numberOf1Bit = 0;
+        while(n) {
+            if(n % 2) {
+                numberOf1Bit += 1;
+            }
+
+            n = n >> 1;
+        }
+
+        return numberOf1Bit;
+    }
+};
+

palindromic-substrings/heozeop.cpp

@@ -0,0 +1,71 @@
+// Time Complexity: O(n^3)
+// Spatial Complexity: O(1)
+
+// #include <set>
+// #include <string>
+
+class Solution {
+private:
+    bool isPalindrom(string s) {
+        int sLength = s.length();
+        
+        for(int i = 0; i < sLength / 2; ++i) {
+            if(s[i] != s[sLength - 1 - i]) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+public:
+    int countSubstrings(string s) {
+        int answer = 0;
+
+        int sLength = s.length();
+        string temp;
+        for(int i = 1; i <= sLength; ++i) {
+            for(int j = 0; j + i <= sLength; ++j) {
+                temp = s.substr(j, i);
+
+                if(this->isPalindrom(temp)) {
+                    ++answer;
+                }
+            }
+        }
+
+        return answer;
+    }
+};
+
+// DP version
+// O(n^2)
+// #include <set>
+// #include <vector>
+
+class Solution2 {
+public:
+    int countSubstrings(string str) {
+        int answer = 0;
+
+        int n = str.length();
+        vector<vector<bool>> dp(n + 1, vector<bool>(n + 1, false));
+
+        for(int e = 0; e < n; ++e) {
+            for(int s = 0; s <= e; ++s) {
+                if (str[s] != str[e]) {
+                    continue;
+                }
+
+                if (s != e && s + 1 != e && !dp[s + 1][e - 1]) {
+                    continue;
+                }
+
+                ++answer;
+                dp[s][e] = true;
+            }
+        }
+
+        return answer;
+    }
+};

top-k-frequent-elements/heozeop.cpp

@@ -0,0 +1,38 @@
+// Time complexity O(nlogn)
+// Spatal Complexity O(n)
+
+#include <map>
+#include <vector>
+
+using namespace std;
+
+
+bool cmp(const pair<int,int>& a, const pair<int,int>& b) {
+    return a.second > b.second;
+}
+
+class Solution {
+public:
+    vector<int> topKFrequent(vector<int>& nums, int k) {
+        map<int, int> countMap;
+
+        for(int num : nums) {
+            if(countMap.find(num) == countMap.end()) {
+                countMap.insert({num, 0});
+            }
+            
+            ++countMap[num];
+        }
+
+        vector<pair<int,int>> vec(countMap.begin(), countMap.end());
+        sort(vec.begin(), vec.end(), cmp);
+
+        vector<int> answer;
+        for(int i = 0; i < k; ++i) {
+            answer.push_back(vec[i].first);
+        }
+
+        return answer;
+    }
+};
+