message: add a solution to README.md
diff: 378 | [Kth Smallest Element in a Sorted Matrix](https://leetcode.com/problems/kth-smallest-element-in-a-sorted-matrix/) | [C++](./C++/kth-smallest-element-in-a-sorted-matrix.cpp) [Python](./Python/kth-smallest-element-in-a-sorted-matrix.py) | _O(k * log(min(n, m, k)))_ | _O(min(n, m, k))_ | Medium | LintCode ||
  407 | [Trapping Rain Water II](https://leetcode.com/problems/trapping-rain-water-ii/) | [C++](./C++/trapping-rain-water-ii.cpp) [Python](./Python/trapping-rain-water-ii.py) | _O(m * n * (logm + logn))_ | _O(m * n)_ | Hard | LintCode ||
  632 | [Smallest Range](https://leetcode.com/problems/smallest-range/) | [C++](./C++/smallest-range.cpp) [Python](./Python/smallest-range.py) | _O(nlogk)_ | _O(k)_ | Hard |||
  +703 | [Kth Largest Element in a Stream](https://leetcode.com/problems/kth-largest-element-in-a-stream/) | [C++](./C++/kth-largest-element-in-a-stream.cpp) [Python](./Python/kth-largest-element-in-a-stream.py) | _O(nlogk)_ | _O(k)_ | Easy |||
  846 | [Hand of Straights](https://leetcode.com/problems/hand-of-straights/) | [C++](./C++/hand-of-straights.cpp) [Python](./Python/hand-of-straights.py) | _O(nlogn)_ | _O(n)_ | Medium |||
  855 | [Exam Room](https://leetcode.com/problems/exam-room/) | [C++](./C++/exam-room.cpp) [Python](./Python/exam-room.py) | seat: _O(logn)_ <br> leave: _O(logn)_ | _O(n)_ | Medium || BST, Hash |
  857 | [Minimum Cost to Hire K Workers](https://leetcode.com/problems/minimum-cost-to-hire-k-workers/) | [C++](./C++/minimum-cost-to-hire-k-workers.cpp) [Python](./Python/minimum-cost-to-hire-k-workers.py) | _O(nlogn)_ | _O(n)_ | Hard || Sort |
  