foo.c: In function 'main':
foo.c:10:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d %d", &m, &n); //m行, n列
  ^~~~~~~~~~~~~~~~~~~~~~
foo.c:16:4: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
    scanf("%d", &a[i][j]); //把点海拔存入二维数组
    ^~~~~~~~~~~~~~~~~~~~~

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