求1+2+3+…+n的和，这种题我也是醉了。给个不一样的解法：假设S2n=1+2+3+…+2nEn=2+4+…+2n，On=1+3+…+2n-1，则有On+n=EnS2n=En+On=2*En-n=4*Sn-n，这样把1~2n的求和变成1~n的求和，问题规模降低了一半，时间复杂度变成O(lgn)
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