可以用来估算运行时间，以及一些题目可能用到
1 :2 2 :4 3 :8 4 :16 5 :32 6 :64 7 :128 8 :256 9 :512 10 :1024 11 :2048 12 :4096 13 :8192 14 :16384 15 :32768 16 :65536 17 :131072 18 :262144 19 :524288 20 :1048576 21 :2097152 22 :4194304 23 :8388608 24 :16777216 25 :33554432 26 :67108864 27 :134217728 28 :268435456 29 :536870912 30 :1073741824 31 :2147483648 32 :4294967296 33 :8589934592 34 :17179869184 35 :34359738368 36 :68719476736 37 :137438953472 38 :274877906944 39 :549755813888 40 :1099511627776 41 :2199023255552 42 :4398046511104 43 :8796093022208 44 :17592186044416 45 :35184372088832 46 :70368744177664 47 :140737488355328 48 :281474976710656 49 :562949953421312 50 :1125899906842624 51 :2251799813685248 52 :4503599627370496 53 :9007199254740992 54 :18014398509481984 55 :36028797018963968 56 :72057594037927936 57 :144115188075855872 58 :288230376151711744 59 :576460752303423488 60 :1152921504606846976 61 :2305843009213693952 62 :4611686018427387904