02- sparse array

1. scenario

• 331 procedures write, save and exit and have continued on disk function
  

• [Analysis] because a lot of the value of the two-dimensional array is the default value 0, and therefore makes no sense to record a lot of data → sparse array

2. Basic Introduction

• When most of the elements in an array is 0, or a value of the same array, the sparse array can be used to hold the array
• Sparse array processing method
  1. An array of records, a total of several odd row, how many different values
  2. And the ranks of the value of records with different values ​​of the elements in an array of small-scale, thereby reducing the size of the program
     

3. Analysis of ideas

→ sparse array of two-dimensional array

1. Traversing the original two-dimensional array, the number of valid data obtained sum
2. The sum can create a sparse array {sparseArr} - int [sum + 1] [3]
3. The two-dimensional array of valid data stored in the sparseArr

→ sparse array of two-dimensional arrays

1. SparseArr first read the first line L1 according to the data to create original two-dimensional array {arr} - int [sparseArr [0] [0]] [sparseArr [0] [1]]
2. After a few lines of the re-read data sparse array, and in accordance with the respective position (c1, c2) assigned to the c3 of arr

4. The code implementation

// 创建一个原始的二维数组 11 * 11
// 0 表示没有棋子; 1 表示黑子; 2 表示白子
public class SparseArrayDemo {
    public static void printArr(int[][] arr) {
        for(int[] row : arr) {
            for(int data : row)
                System.out.printf("%d\t", data);
            System.out.println();
        }
    }
    
    public static void main(String[] args) {
        // 0. 创建棋盘, 摆好棋子
        int chessArr1[][] = new int[11][11];
        chessArr1[1][2] = 1;
        chessArr1[2][3] = 2;
        System.out.println("棋盘：");
        printArr(chessArr1);
        
        // 1. [存盘]二维数组 → 稀疏数组
        // 1.1 遍历二维数组, 得到 !0 数据的个数
        int sum = 0;
        int row = chessArr1.length, column = chessArr1[0].length;
        for(int i = 0; i < row; i++)
            for(int j = 0; j < column; j++)
                if(chessArr1[i][j] != 0)
                    sum++;
        System.out.println("sum = " + sum);
        // 1.2 创建对应的稀疏数组
        int sparseArr[][] = new int[sum+1][3];
        // 给稀疏数组赋值
        // L-1
        sparseArr[0][0] = row;
        sparseArr[0][1] = column;
        sparseArr[0][2] = sum;
        // L-2 ~ L-sum
        // 遍历二维数组, 将 !0 值放入稀疏数组中
        int line = 1;
        for(int i = 0; i < row; i++)
            for(int j = 0; j < column; j++)
                if(chessArr1[i][j] != 0) {
                    sparseArr[line][0] = i;
                    sparseArr[line][1] = j;
                    sparseArr[line][2] = chessArr1[i][j];
                    line++;
                }
        System.out.println("存盘：");
        printArr(sparseArr);
        
        // 2. [复盘]稀疏数组 → 二维数组
        // 2.1 读取sparseArr的第一行, 创建棋盘
        int chessArr2[][] = new int[sparseArr[0][0]][sparseArr[0][1]];
        // 2.2 读取 L-2 ~ L-sum, 恢复棋盘
        for(int i = 1; i <= sparseArr[0][2]; i++)
            chessArr2[sparseArr[i][0]][sparseArr[i][1]] = sparseArr[i][2];
        System.out.println("复盘：");
        printArr(chessArr2);
    }
}