a1,b2,c5,d10 ----- 开始, sum = 0!<br /> c5,d10 ------> a1,b2, sum = 2;<br /> a1,c5,d10 <------ b2, sum = 3;<br /> a1 ------> b2,c5,d10, sum = 13;<br /> a1 ,b2 ------> c5,d10, sum = 15;<br /> ------> a1,b2,c5,d10, sum = 17;<br />
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