Added leetcode solutions (neetcode-gh#1798)

* added leetcode questions

* renamed

* Rename first-missing-positive.cpp to 41-first-missing-positive.cpp

* Rename word-pattern.cpp to 290-word-pattern.cpp

* Rename 41-first-missing-positive.cpp to 0041-first-missing-positive.cpp

* Rename 290-word-pattern.cpp to 0290-word-pattern.cpp

Co-authored-by: Tahsin Tunan <[email protected]>

Author: diva-21

cpp/0041-first-missing-positive.cpp

@@ -1,21 +1,16 @@
-/*
-  Time: O(n)
-  Space: O(1)
-*/
-
-class Solution {
- public:
-  int firstMissingPositive(vector<int>& nums) {
-    const int n = nums.size();
-
-    for (int i = 0; i < n; ++i)
-      while (nums[i] > 0 && nums[i] <= n && nums[i] != nums[nums[i] - 1])
-        swap(nums[i], nums[nums[i] - 1]);
-
-    for (int i = 0; i < n; ++i)
-      if (nums[i] != i + 1)
-        return i + 1;
-
-    return n + 1;
-  }
-};
+class Solution {
+public:
+    int firstMissingPositive(vector<int>& nums) {
+        for(int i=0; i<nums.size(); i++){
+            if(i+1==nums[i]) continue;
+            int x = nums[i];
+            while(x>=1 && x<=nums.size() && x!=nums[x-1]){
+                swap(x, nums[x-1]);
+            }
+        }
+        for(int i=0; i<nums.size(); i++){
+            if(i+1!=nums[i])    return i+1;
+        }
+        return nums.size()+1;
+    }
+};

cpp/0290-word-pattern.cpp

@@ -0,0 +1,19 @@
+class Solution {
+public:
+    bool wordPattern(string pattern, string str) {
+        vector<int> pat_map (26, 0);
+        unordered_map<string,int> str_map;
+        int i=0, n = pattern.size();
+        istringstream ss (str);
+        string token;
+        
+        for(string token; ss >> token; ++i) {
+            if(i == n || pat_map[pattern[i]-'a'] != str_map[token]) return false;
+            
+            // 1-based indexing since map assigns 0 as a default value for keys not found.
+            pat_map[pattern[i]-'a'] = str_map[token] = i+1;
+        }
+        
+        return i == n;
+    }
+};

cpp/1930-unique-length-3-palindromic-subsequences.cpp

@@ -0,0 +1,21 @@
+class Solution {
+public:
+    int countPalindromicSubsequence(string s) {
+        vector<pair<int, int>> v(26, {-1, -1});
+        for (int i = 0; i < s.size(); i++) {
+            if (v[s[i] - 'a'].first == -1) v[s[i] - 'a'].first = i;
+            else v[s[i] - 'a'].second = i;
+        }
+        
+        int res = 0;
+        for (int i = 0; i < 26; i++) {
+            if (v[i].second != -1) {
+                unordered_set<char> tmp;
+                for (int j = v[i].first + 1; j < v[i].second; j++) tmp.insert(s[j]);
+                res += tmp.size();
+            }
+        }
+        
+        return res;
+    }
+};