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2387.Median-of-a-Row-Wise-Sorted-Matrix

令k=(m*n+1)/2，本题就是求矩阵里的从小到大的第k个元素。本质和215.Kth-Largest-Element-in-an-Array相同，只不过需要将各行smallerOrEqual(mid)的结果累加起来得到count。

if (count < k)
  left = mid+1;
else
  right = mid;