leetcode-289

Author: maslke

Array.md

@@ -6,6 +6,7 @@ No.|Title|Difficulty|Solved|Date
 26|[Remove Duplicates from Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/)|Easy|yes|2019-01-10
 27|[Remove Element](https://leetcode.com/problems/remove-element/)|Easy|yes|2019-01-10
 153|[Find Minimum in Rotated Sorted Array](https://leetcode.com/problems/find-minimum-in-rotated-sorted-array/)|Medium|yes|2019-01-19
+289|[Game Of Life](https://leetcode.com/problems/game-of-life/)|Medium|yes|2020-01-11
 791|[Custom Sort String](https://leetcode.com/problems/custom-sort-string/)|Medium|yes|2020-01-07
 ----------------------------------------------------------------
 1. [Two Sum](https://leetcode.com/problems/two-sum/)
@@ -46,6 +47,12 @@ private int findMin(int[] nums, int start, int end) {
     }
 ```  
 
+289. [Game Of Life](https://leetcode.com/problems/game-of-life/)
+
+要求不使用额外变量临时记录中间状态的话，考虑使用特征值的方法。所有的Cell都处理完成之后，再将特征值替换会原始的值。
+
+例如，死亡的Cell -> 存活的Cell，这一过程的特征值为 2。存活的Cell -> 死亡的Cell，这一过程的特征值为 -1。在统计Cell周边存活Cell的个数的时候，需要统计值为-1的Cell。
+
 791. [Custom Sort String](https://leetcode.com/problems/custom-sort-string/)
 
 自定义排序，排序规则是在给定的字符串中。

src/main/java/medium/lc289.java

@@ -0,0 +1,67 @@
+package medium;
+
+class lc289 {
+    // (0 -> 1) -> 2
+    // (1 -> 0) -> -1
+    public void gameOfLife(int[][] board) {
+
+        for (int i = 0; i < board.length; i++) {
+            for (int j = 0; j < board[i].length; j++) {
+                int aliveCount = getAliveCount(board, i, j);
+                if (board[i][j] == 0) {
+                    if (aliveCount == 3) {
+                        board[i][j] = 2;
+                    }
+                } else if (board[i][j] == 1) {
+                    if (aliveCount < 2 || aliveCount > 3) {
+                        board[i][j] = -1;
+                    }
+                }
+            }
+        }
+
+        for (int i = 0; i < board.length; i++) {
+            for (int j = 0; j < board[i].length; j++) {
+                if (board[i][j] == -1) {
+                    board[i][j] = 0;
+                } else if (board[i][j] == 2) {
+                    board[i][j] = 1;
+                }
+            }
+        }
+    }
+
+    private boolean isAlive(int[][] board, int i, int j) {
+        return board[i][j] == 1 || board[i][j] == -1;
+    }
+
+    private int getAliveCount(int[][] board, int i, int j) {
+        int result = 0;
+        if (i - 1 >= 0 && j - 1 >= 0 && isAlive(board, i - 1, j - 1)) {
+            result += 1;
+        }
+        if (i - 1 >= 0 && isAlive(board, i - 1, j)) {
+            result += 1;
+        }
+        if (i - 1 >= 0 && j + 1 < board[i].length && isAlive(board, i - 1, j + 1)) {
+            result += 1;
+        }
+        if (j - 1 >= 0 && isAlive(board, i, j - 1)) {
+            result += 1;
+        }
+
+        if (j + 1 < board[i].length && isAlive(board, i, j + 1)) {
+            result += 1;
+        }
+        if (i + 1 < board.length && j - 1 >= 0 && isAlive(board, i + 1, j - 1)) {
+            result += 1;
+        }
+        if (i + 1 < board.length && isAlive(board, i + 1, j)) {
+            result += 1;
+        }
+        if (i + 1 < board.length && j + 1 < board[i].length && isAlive(board, i + 1, j + 1)) {
+            result += 1;
+        }
+        return result;
+    }
+}