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倒数第二个踩入点；
寻找最后一个踩入点ifi20j10boardi2j10ii2jj1elseifi20j1Mboardi2j10ii2jj1elseifi10j2Mboardi1j20ii1jj2elseifi1Mj2Mboardi1j20ii1jj2elseifj2Mi1Mboardi2j10ii2jj1elseifi2Mj10boardi2j10ii2
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