# 二维数组（矩阵）对角线输出

```
/**

{ 1, 2, 3, 4 },
{ 5, 6, 7, 8 },
{ 9, 10, 11, 12 },
{ 13, 14, 15, 16 },

slash方向输出：
1
5 2
9 6 3
13 10 7 4
14 11 8
15 12
16

backslash输出：
4
3 8
2 7 12
1 6 11 16
5 10 15
9 14
13

*/
public class PrintArray {

public static void main(String[] args) {
/*// row != col:
int[][] arr = new int[][] {
{ 1, 2, 3, 4 },
{ 5, 6, 7, 8 },
{ 9, 10, 11, 12 },
//{ 13, 14, 15, 16 },
};
*/
int[][] arr = new int[][] {
{ 1, 2, 3, 4 },
{ 5, 6, 7, 8 },
{ 9, 10, 11, 12 },
{ 13, 14, 15, 16 },
};

slash(arr);
System.out.println("======");
backslash(arr);

}

public static void slash(int[][] arr) {
if (arr == null) {
throw new IllegalArgumentException("arr cannot be null");
}
int col = arr[0].length;
int row = arr.length;
for( int k = 0 ; k < col * 2 -1 ; k++ ) {     //k代表有多少行输出，由col决定
int sum = k;                              //同一行的元素，行下标和列下标的和相等
for( int j = 0 ; j < col ; j++ ) {
int i = sum - j;
if(isValidIndex(i, row) && isValidIndex(j, col)) {
System.out.print( arr[i][j] + " " );
}
}
System.out.println();
}
}

public static void backslash(int[][] arr) {
if (arr == null) {
throw new IllegalArgumentException("arr cannot be null");
}
int row = arr.length;
int col = arr[0].length;
for (int k = 0; k < col * 2 - 1; k++) {     //k代表有多少行输出，由col决定
int diff = col - k - 1;                 //同一行的元素，行下标和列下标的差相等
for (int j = 0; j < col; j++) {
int i = j - diff;
if (isValidIndex(i, row) && isValidIndex(j, col)) {
System.out.print(arr[i][j] + " ");
}
}
System.out.println();
}
}

private static boolean isValidIndex(int i, int n) {
return i >= 0 && i < n;
}

}
```

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