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Leetcode 289. Game of Life

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Question:

According to the Wikipedia's article: "The Game of Life, also known simply as Life, is a cellular automaton devised by the British mathematician John Horton Conway in 1970."

Given a board with m by n cells, each cell has an initial state live (1) or dead (0). Each cell interacts with its eight neighbors (horizontal, vertical, diagonal) using the following four rules (taken from the above Wikipedia article):

Any live cell with fewer than two live neighbors dies, as if caused by under-population. Any live cell with two or three live neighbors lives on to the next generation. Any live cell with more than three live neighbors dies, as if by over-population.. Any dead cell with exactly three live neighbors becomes a live cell, as if by reproduction. Write a function to compute the next state (after one update) of the board given its current state.

Follow up: Could you solve it in-place? Remember that the board needs to be updated at the same time: You cannot update some cells first and then use their updated values to update other cells. In this question, we represent the board using a 2D array. In principle, the board is infinite, which would cause problems when the active area encroaches the border of the array. How would you address these problems?

Explanation:

依次遍历matrix，判断周围八个方向总和，如果之后是live，就设置为3，否则设置为2。最后再遍历一次，改成0或者1。 感觉这道题的考点是in place，其他的比较简单，遍历没有讨巧的方法。

Code:

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public class Solution {
    public void gameOfLife(int[][] board) {
        if (board == null || board.length == 0) return;
        int m = board.length; int n = board[0].length;
        for (int i=0; i<board.length; ++i) {
            for (int j = 0; j < board[0].length; ++j) {
                int sum = sumHelper(board, i, j, m, n);
                if(board[i][j]==0 && sum==3) board[i][j]=3;
                if(board[i][j]==1 && (sum<2 || sum>3)) board[i][j]=2;
            }
        }
        for (int i=0; i<board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                board[i][j]%=2;
            }
        }
    }
    public int sumHelper(int[][] board, int i, int j, int m, int n) {
        int sum = 0;
        for (int x = Math.max(i-1, 0); x<=Math.min(i+1, m-1); ++x) {
            for (int y = Math.max(j-1,0); y<=Math.min(j+1,n-1); ++y) {
                if (board[x][y] == 2 || board[x][y] == 1) {
                    sum += 1;
                }
            }
        }
        sum -= board[i][j];
        return sum;
    }
}