add 877

Author: fishercoder1534

README.md

@@ -392,6 +392,7 @@ _If you like this project, please leave me a star._ ★
 |883|[Projection Area of 3D Shapes](https://leetcode.com/problems/projection-area-of-3d-shapes/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_883.java) | |Easy|Math
 |881|[Boats to Save People](https://leetcode.com/problems/boats-to-save-people/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_881.java) | |Medium|Two Pointers, Greedy
 |880|[Decoded String at Index](https://leetcode.com/problems/decoded-string-at-index/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_880.java) | |Medium|Stack
+|877|[Stone Game](https://leetcode.com/problems/stone-game/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_877.java) | |Medium| Math, DP, Minimax
 |876|[Middle of the Linked List](https://leetcode.com/problems/middle-of-the-linked-list/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_876.java) | |Easy|
 |872|[Leaf-Similar Trees](https://leetcode.com/problems/leaf-similar-trees/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_872.java) |  |Easy| DFS, recursion
 |868|[Binary Gap](https://leetcode.com/problems/binary-gap/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_868.java) |  |Easy|

src/main/java/com/fishercoder/solutions/_877.java

@@ -0,0 +1,41 @@
+package com.fishercoder.solutions;
+
+import java.util.Arrays;
+
+public class _877 {
+    public static class Solution1 {
+        /**
+         * credit: https://leetcode.com/problems/stone-game/discuss/154660/Java-This-is-minimax-%2B-dp-(fully-detailed-explanation-%2B-generalization-%2B-easy-understand-code)
+         * <p>
+         * Suppose the ID for Alex is 1, that for Lee is 0
+         * Alex wants to maximize the score to win while Lee wants to minimize the score to win.
+         * Each time, each player has two options to pick, we'll use recursion to find the most optimal choice for each of them.
+         */
+        public boolean stoneGame(int[] piles) {
+            int len = piles.length;
+            int[][][] dp = new int[len + 1][len + 1][2];
+            for (int[][] arr : dp) {
+                for (int[] num : arr) {
+                    Arrays.fill(num, -1);
+                }
+            }
+            return recursion(dp, 0, len - 1, 1, piles) > 0;
+        }
+
+        private int recursion(int[][][] dp, int left, int right, int identifier, int[] piles) {
+            if (left > right) {
+                return 0;
+            }
+            if (dp[left][right][identifier] != -1) {
+                return dp[left][right][identifier];
+            }
+            int next = Math.abs(identifier - 1);
+            if (identifier == 1) {
+                dp[left][right][identifier] = Math.max(piles[left] + recursion(dp, left + 1, right, next, piles), piles[right] + recursion(dp, left, right - 1, next, piles));
+            } else {
+                dp[left][right][identifier] = Math.min(-piles[left] + recursion(dp, left + 1, right, next, piles), -piles[right] + recursion(dp, left, right - 1, next, piles));
+            }
+            return dp[left][right][identifier];
+        }
+    }
+}

src/test/java/com/fishercoder/_877Test.java

@@ -0,0 +1,22 @@
+package com.fishercoder;
+
+import com.fishercoder.solutions._877;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class _877Test {
+    private static _877.Solution1 solution1;
+
+    @BeforeClass
+    public static void setup() {
+        solution1 = new _877.Solution1();
+    }
+
+    @Test
+    public void test1() {
+        assertEquals(true, solution1.stoneGame(new int[]{5, 3, 4, 5}));
+    }
+
+}