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int[][] matrix;

/* 异常情况：null [] [[]]
** 否则matrix[0]会出现空指针的错误
*/
if (matrix == null || matrix.length == 0) {
    return false;
}
if (matrix[0] == null || matrix[0].length == 0) {
    return false;
}
int m = matrix.length;
int n = matrix[0].length;