add 679

Author: fishercoder1534

README.md

@@ -23,6 +23,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |  #  |      Title     |   Solutions   | Time          | Space         | Difficulty  | Tag          | Notes
 |-----|----------------|---------------|---------------|---------------|-------------|--------------|-----
 |680|[Valid Palindrome II](https://leetcode.com/problems/valid-palindrome-ii/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_680.java) | O(n) | O(1) | Easy | String
+|679|[24 Game](https://leetcode.com/problems/24-game/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_679.java) | O(n^2) | O(n) | Hard | Recursion
 |677|[Map Sum Pairs](https://leetcode.com/problems/map-sum-pairs/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_677.java) | O(n) | O(n) | Medium | HashMap
 |676|[Implement Magic Dictionary](https://leetcode.com/problems/implement-magic-dictionary/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_676.java) | O(n^2) | O(n) | Medium |
 |674|[Longest Continuous Increasing Subsequence](https://leetcode.com/problems/longest-continuous-increasing-subsequence/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_674.java) | O(n^2) | O(1) | Easy | 

src/main/java/com/fishercoder/solutions/_679.java

@@ -0,0 +1,77 @@
+package com.fishercoder.solutions;
+
+import java.util.stream.IntStream;
+
+/**
+ * 679. 24 Game
+ *
+ *  You have 4 cards each containing a number from 1 to 9.
+ *  You need to judge whether they could operated through *, /, +, -, (, ) to get the value of 24.
+
+ Example 1:
+ Input: [4, 1, 8, 7]
+ Output: True
+ Explanation: (8-4) * (7-1) = 24
+
+ Example 2:
+ Input: [1, 2, 1, 2]
+ Output: False
+
+ Note:
+ The division operator / represents real division, not integer division. For example, 4 / (1 - 2/3) = 12.
+ Every operation done is between two numbers.
+ In particular, we cannot use - as a unary operator. For example, with [1, 1, 1, 1] as input, the expression -1 - 1 - 1 - 1 is not allowed.
+ You cannot concatenate numbers together. For example, if the input is [1, 2, 1, 2], we cannot write this as 12 + 12.
+
+ */
+
+public class _679 {
+    public static class Solution1 {
+        public boolean judgePoint24(int[] nums) {
+            return dfs(IntStream.of(nums).mapToDouble(num -> num).toArray());
+        }
+
+        private boolean dfs(double[] nums) {
+            if (nums.length == 1) {
+                return Math.abs(nums[0] - 24) < 1e-8;//1e-8 means 0.000000001, i.e. 10^(-8)
+            }
+
+            for (int i = 0; i < nums.length; i++) {
+                for (int j = 0; j < nums.length; j++) {
+                    if (i != j) {
+                        int len = 0;
+                        double[] a = new double[nums.length-1];
+                        for (int k = 0; k < nums.length; k++) {
+                            if (k != i && k != j) {
+                                a[len++] = nums[k];
+                            }
+                        }
+
+                        a[len] = nums[i] + nums[j];
+                        if (dfs(a)) {
+                            return true;
+                        }
+
+                        a[len] = nums[i] - nums[j];
+                        if (dfs(a)) {
+                            return true;
+                        }
+
+                        a[len] = nums[i] * nums[j];
+                        if (dfs(a)) {
+                            return true;
+                        }
+
+                        if (nums[j] > 1e-8) {
+                            a[len] = nums[i] / nums[j];
+                            if (dfs(a)) {
+                                return true;
+                            }
+                        }
+                    }
+                }
+            }
+            return false;
+        }
+    }
+}

src/test/java/com/fishercoder/_679Test.java

@@ -0,0 +1,43 @@
+package com.fishercoder;
+
+import com.fishercoder.solutions._679;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class _679Test {
+    private static _679.Solution1 solution1;
+
+    @BeforeClass
+    public static void setup() {
+        solution1 = new _679.Solution1();
+    }
+
+    @Test
+    public void test1() {
+        assertEquals(true, solution1.judgePoint24(new int[]{4,1,8,7}));
+    }
+
+    @Test
+    public void test2() {
+        assertEquals(false, solution1.judgePoint24(new int[]{1,2,1,2}));
+    }
+
+    @Test
+    public void test3() {
+//        8 / (1 - 2/3) = 24
+        assertEquals(true, solution1.judgePoint24(new int[]{1,2,3,8}));
+    }
+
+    @Test
+    public void test4() {
+        assertEquals(true, solution1.judgePoint24(new int[]{1,3,4,6}));
+    }
+
+    @Test
+    public void test5() {
+        assertEquals(true, solution1.judgePoint24(new int[]{1,9,1,2}));
+    }
+
+}