Add 2 problems

Author: ex01tus

README.md

@@ -2,7 +2,7 @@
 
 ![DSA Learning](https://img.shields.io/badge/DSA-Learning-blue?style=for-the-badge&logo=leetcode)
 
-This repository contains solutions for 588 DSA problems from the [LeetCode](https://leetcode.com/)
+This repository contains solutions for 590 DSA problems from the [LeetCode](https://leetcode.com/)
 website.
 
 Problems are separated into 20 topics:
@@ -22,12 +22,12 @@ Problems are separated into 20 topics:
 | 11 | Interval            | [interval](src/interval)                       |       12 |
 | 12 | Linked List         | [linked_list](src/linked_list)                 |       34 |
 | 13 | Math                | [math](src/math)                               |       23 |
-| 14 | Matrix              | [matrix](src/matrix)                           |       19 |
+| 14 | Matrix              | [matrix](src/matrix)                           |       20 |
 | 15 | Quad Tree           | [quad_tree](src/quad_tree)                     |        1 |
 | 16 | Recursion           | [recursion](src/recursion)                     |       24 |
 | 17 | SQL                 | [sql](src/sql)                                 |       28 |
 | 18 | Stack               | [stack](src/stack)                             |       25 |
-| 19 | String              | [string](src/string)                           |       61 |
+| 19 | String              | [string](src/string)                           |       62 |
 | 20 | Trie                | [trie](src/trie)                               |        4 |
 
 ## Good places to start your own LeetCode journey

src/matrix/FindSmallestCommonElementInAllRows.java

@@ -0,0 +1,25 @@
+package matrix;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Description: https://leetcode.com/problems/find-smallest-common-element-in-all-rows
+ * Difficulty: Medium
+ * Time complexity: O(n * m)
+ * Space complexity: O(n * m)
+ */
+public class FindSmallestCommonElementInAllRows {
+
+    public int smallestCommonElement(int[][] matrix) {
+        Map<Integer, Integer> freqMap = new HashMap<>();
+        for (int i = 0; i < matrix.length; i++) {
+            for (int j = 0; j < matrix[0].length; j++) {
+                int count = freqMap.merge(matrix[i][j], 1, Integer::sum);
+                if (count == matrix.length) return matrix[i][j];
+            }
+        }
+
+        return -1;
+    }
+}

src/string/PerformStringShifts.java

@@ -0,0 +1,25 @@
+package string;
+
+/**
+ * Description: https://leetcode.com/problems/perform-string-shifts
+ * Difficulty: Easy
+ * Time complexity: O(n)
+ * Space complexity: O(n)
+ */
+public class PerformStringShifts {
+
+    public String stringShift(String s, int[][] shifts) {
+        int netShift = 0;
+        for (int[] shift : shifts) {
+            int direction = shift[0] == 0 ? -1 : 1;
+            netShift += direction * shift[1];
+        }
+
+        netShift %= s.length();
+        if (netShift == 0) return s;
+
+        return netShift < 0
+                ? s.substring(-netShift) + s.substring(0, -netShift) // move left
+                : s.substring(s.length() - netShift) + s.substring(0, s.length() - netShift); // move right
+    }
+}