add a solution for 224

Author: fishercoder1534

README.md

@@ -1220,7 +1220,7 @@ _If you like this project, please leave me a star._ ★
 | 227  | [Basic Calculator II](https://leetcode.com/problems/basic-calculator-ii/)                                                                                                                     | [Solution](../master/src/main/java/com/fishercoder/solutions/_227.java)                                                                  |                                                                               | Medium                           | String                                                               
 | 226  | [Invert Binary Tree](https://leetcode.com/problems/invert-binary-tree/)                                                                                                                       | [Solution](../master/src/main/java/com/fishercoder/solutions/_226.java)                                                                  |                                                                               | Easy                             | DFS, recursion                                                       
 | 225  | [Implement Stack using Queues](https://leetcode.com/problems/implement-stack-using-queues/)                                                                                                   | [Solution](../master/src/main/java/com/fishercoder/solutions/_225.java)                                                                  |                                                                               | Easy                             | Stack, Queue                                                         
-| 224  | [Basic Calculator](https://leetcode.com/problems/basic-calculator/)                                                                                                                           | [Solution](../master/src/main/java/com/fishercoder/solutions/_224.java)                                                                  |                                                                               | Hard                             | 
+| 224  | [Basic Calculator](https://leetcode.com/problems/basic-calculator/)                                                                                                                           | [Solution](../master/src/main/java/com/fishercoder/solutions/_224.java)                                                                  |                                                                               | Hard                             | Recursion 
 | 223  | [Rectangle Area](https://leetcode.com/problems/rectangle-area/)                                                                                                                               | [Solution](../master/src/main/java/com/fishercoder/solutions/_223.java)                                                                  |                                                                               | Easy                             | 
 | 222  | [Count Complete Tree Nodes](https://leetcode.com/problems/count-complete-tree-nodes/)                                                                                                         | [Solution](../master/src/main/java/com/fishercoder/solutions/_222.java)                                                                  |                                                                               | Medium                           | Recursion                                                            
 | 221  | [Maximal Square](https://leetcode.com/problems/maximal-square/)                                                                                                                               | [Solution](../master/src/main/java/com/fishercoder/solutions/_221.java)                                                                  |                                                                               | Medium                           | Recursion                                                            

src/main/java/com/fishercoder/solutions/_224.java

@@ -68,7 +68,45 @@ public int calculate(String s) {
             }
             return !stack.isEmpty() ? Integer.parseInt(stack.peekLast()) + result : result;
         }
+    }
+
+    public static class Solution2 {
+        /**
+         * Simple and clean recursion solution, credit: https://leetcode.com/problems/basic-calculator/solutions/2344042/java-2ms-100-recursion-easy-to-understand/
+         * Key points:
+         * 1. it uses a global variable called index to control which char to iterate on;
+         * 2. it passes the entire string s into recursive functions.
+         */
+        int index;
 
+        public int calculate(String s) {
+            index = 0;
+            return cal(s);
+        }
+
+        private int cal(String s) {
+            int result = 0;
+            int num = 0;
+            int sign = 1;
+            while (index < s.length()) {
+                char c = s.charAt(index++);
+                if (c >= '0' && c <= '9') {
+                    num = num * 10 + c - '0';
+                } else if (c == '(') {
+                    //this is the beginning of a new sub-problem, we let recursion do its job
+                    num = cal(s);
+                } else if (c == ')') {
+                    //this is the end of a problem/sub-problem, so we return
+                    return result + sign * num;
+                } else if (c == '+' || c == '-') {
+                    //now we know we finished reading one number and a new number has begun
+                    result += sign * num;
+                    num = 0;
+                    sign = c == '-' ? -1 : 1;
+                }
+            }
+            return result + sign * num;
+        }
     }
 
 }

src/test/java/com/fishercoder/_224Test.java

@@ -1,46 +1,52 @@
 package com.fishercoder;
 
 import com.fishercoder.solutions._224;
-import org.junit.BeforeClass;
-import org.junit.Test;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
 
-import static org.junit.Assert.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 
 public class _224Test {
     private static _224.Solution1 solution1;
+    private static _224.Solution2 solution2;
     private static int expected;
 
-    @BeforeClass
-    public static void setup() {
+    @BeforeEach
+    public void setup() {
         solution1 = new _224.Solution1();
+        solution2 = new _224.Solution2();
     }
 
     @Test
     public void test1() {
         String s = "1 + 1";
         expected = 2;
         assertEquals(expected, solution1.calculate(s));
+        assertEquals(expected, solution2.calculate(s));
     }
 
     @Test
     public void test2() {
         String s = " 2-1 + 2 ";
         expected = 3;
         assertEquals(expected, solution1.calculate(s));
+        assertEquals(expected, solution2.calculate(s));
     }
 
     @Test
     public void test3() {
         String s = "(1+(4+5+2)-3)+(6+8)";
         expected = 23;
         assertEquals(expected, solution1.calculate(s));
+        assertEquals(expected, solution2.calculate(s));
     }
 
     @Test
     public void test4() {
         String s = "1-(-2)";
         expected = 3;
         assertEquals(expected, solution1.calculate(s));
+        assertEquals(expected, solution2.calculate(s));
     }
 
 }