原文链接 http://reborncodinglife.com/2016/01/17/algorithm-complexity/
注：以下为加速网络访问所做的原文缓存，经过重新格式化，可能存在格式方面的问题，或偶有遗漏信息，请以原文为准。
本文主要通过介绍如何计算十进制数转换成二进制数后，其二进制数中是1的个数,进而分析算法复杂度相关问题。例如十进制数7,二进制表示为0111,总共有三个1。
代码使用go语言实现，为简单起见，算法4和算法5只能计算0-255范围之内的数。
算法1
算法复杂度是O(N),其中N是十进制数字的二进制表示位数。 比如：十进制16，二进制表示为：1 0000 计算出16二进制数中1的个数需运算5次。
func divideCount(number int) int {
counter := 0
for counter = 0; number != 0; number /= 2 {
if number%2 == 1 {
counter++
}
}
return counter
}
算法2
算法复杂度是O(N),其中N是十进制数字的二进制表示位数。 比如：十进制16，二进制表示为：1 0000 计算出16二进制数中1的个数需运算5次。
func shiftCount(number int) int {
counter := 0
for counter = 0; number != 0; number >>= 1 {
if number&1 != 0 {
counter++
}
}
return counter
}
算法3
算法复杂度是O(N),其中N是十进制数字的二进制位数中是1的位数。 比如：十进制16，二进制表示为：1 0000 计算出16的二进制数中1的个数需运算1次（一个1）。 十进制15，二进制表示为：1111 计算出15的二进制数中1的个数需运算4次（4个1）。
func subtractCount(a int) int {
counter := 0
for counter = 0; a != 0; a &= (a - 1) {
counter++
}
return counter
}
算法4
算法复杂度是一个动态值，范围是O(1)至O(1)*255,因为如果是数字0，则运算一次就得出结果。
如果是数字255，需经过255次运算才能得出结果（因为需要执行255次case语句）。
比如：计算出0的二进制数中1的个数只需运算1次，
而计算出255的二进制数中1的个数需运算255次。
func switchCount(number int) int {
counter := 0
switch number {
case 0:
counter = 0
case 1:
fallthrough
case 2:
fallthrough
case 4:
fallthrough
case 8:
fallthrough
case 16:
fallthrough
case 32:
fallthrough
case 64:
fallthrough
case 128:
counter = 1
case 3:
fallthrough
case 5:
fallthrough
case 6:
fallthrough
case 9:
fallthrough
case 10:
fallthrough
case 12:
fallthrough
case 17:
fallthrough
case 18:
fallthrough
case 20:
fallthrough
case 24:
fallthrough
case 33:
fallthrough
case 34:
fallthrough
case 36:
fallthrough
case 40:
fallthrough
case 48:
fallthrough
case 65:
fallthrough
case 66:
fallthrough
case 68:
fallthrough
case 72:
fallthrough
case 80:
fallthrough
case 96:
fallthrough
case 129:
fallthrough
case 130:
fallthrough
case 132:
fallthrough
case 136:
fallthrough
case 144:
fallthrough
case 160:
fallthrough
case 192:
counter = 2
case 7:
fallthrough
case 11:
fallthrough
case 13:
fallthrough
case 14:
fallthrough
case 19:
fallthrough
case 21:
fallthrough
case 22:
fallthrough
case 25:
fallthrough
case 26:
fallthrough
case 28:
fallthrough
case 35:
fallthrough
case 37:
fallthrough
case 38:
fallthrough
case 41:
fallthrough
case 42:
fallthrough
case 44:
fallthrough
case 49:
fallthrough
case 50:
fallthrough
case 52:
fallthrough
case 56:
fallthrough
case 67:
fallthrough
case 69:
fallthrough
case 70:
fallthrough
case 73:
fallthrough
case 74:
fallthrough
case 76:
fallthrough
case 81:
fallthrough
case 82:
fallthrough
case 84:
fallthrough
case 88:
fallthrough
case 97:
fallthrough
case 98:
fallthrough
case 100:
fallthrough
case 104:
fallthrough
case 112:
fallthrough
case 131:
fallthrough
case 133:
fallthrough
case 134:
fallthrough
case 137:
fallthrough
case 138:
fallthrough
case 140:
fallthrough
case 145:
fallthrough
case 146:
fallthrough
case 148:
fallthrough
case 152:
fallthrough
case 161:
fallthrough
case 162:
fallthrough
case 164:
fallthrough
case 168:
fallthrough
case 176:
fallthrough
case 193:
fallthrough
case 194:
fallthrough
case 196:
fallthrough
case 200:
fallthrough
case 208:
fallthrough
case 224:
counter = 3
case 15:
fallthrough
case 23:
fallthrough
case 27:
fallthrough
case 29:
fallthrough
case 30:
fallthrough
case 39:
fallthrough
case 43:
fallthrough
case 45:
fallthrough
case 46:
fallthrough
case 51:
fallthrough
case 53:
fallthrough
case 54:
fallthrough
case 57:
fallthrough
case 58:
fallthrough
case 60:
fallthrough
case 71:
fallthrough
case 75:
fallthrough
case 77:
fallthrough
case 78:
fallthrough
case 83:
fallthrough
case 85:
fallthrough
case 86:
fallthrough
case 89:
fallthrough
case 90:
fallthrough
case 92:
fallthrough
case 99:
fallthrough
case 101:
fallthrough
case 102:
fallthrough
case 105:
fallthrough
case 106:
fallthrough
case 108:
fallthrough
case 113:
fallthrough
case 114:
fallthrough
case 116:
fallthrough
case 120:
fallthrough
case 135:
fallthrough
case 139:
fallthrough
case 141:
fallthrough
case 142:
fallthrough
case 147:
fallthrough
case 149:
fallthrough
case 150:
fallthrough
case 153:
fallthrough
case 154:
fallthrough
case 156:
fallthrough
case 163:
fallthrough
case 165:
fallthrough
case 166:
fallthrough
case 169:
fallthrough
case 170:
fallthrough
case 172:
fallthrough
case 177:
fallthrough
case 178:
fallthrough
case 180:
fallthrough
case 184:
fallthrough
case 195:
fallthrough
case 197:
fallthrough
case 198:
fallthrough
case 201:
fallthrough
case 202:
fallthrough
case 204:
fallthrough
case 209:
fallthrough
case 210:
fallthrough
case 212:
fallthrough
case 216:
fallthrough
case 225:
fallthrough
case 226:
fallthrough
case 228:
fallthrough
case 232:
fallthrough
case 240:
counter = 4
case 31:
fallthrough
case 47:
fallthrough
case 55:
fallthrough
case 59:
fallthrough
case 61:
fallthrough
case 62:
fallthrough
case 79:
fallthrough
case 87:
fallthrough
case 91:
fallthrough
case 93:
fallthrough
case 94:
fallthrough
case 103:
fallthrough
case 107:
fallthrough
case 109:
fallthrough
case 110:
fallthrough
case 115:
fallthrough
case 117:
fallthrough
case 118:
fallthrough
case 121:
fallthrough
case 122:
fallthrough
case 124:
fallthrough
case 143:
fallthrough
case 151:
fallthrough
case 155:
fallthrough
case 157:
fallthrough
case 158:
fallthrough
case 167:
fallthrough
case 171:
fallthrough
case 173:
fallthrough
case 174:
fallthrough
case 179:
fallthrough
case 181:
fallthrough
case 182:
fallthrough
case 185:
fallthrough
case 186:
fallthrough
case 188:
fallthrough
case 199:
fallthrough
case 203:
fallthrough
case 205:
fallthrough
case 206:
fallthrough
case 211:
fallthrough
case 213:
fallthrough
case 214:
fallthrough
case 217:
fallthrough
case 218:
fallthrough
case 220:
fallthrough
case 227:
fallthrough
case 229:
fallthrough
case 230:
fallthrough
case 233:
fallthrough
case 234:
fallthrough
case 236:
fallthrough
case 241:
fallthrough
case 242:
fallthrough
case 244:
fallthrough
case 248:
counter = 5
case 63:
fallthrough
case 95:
fallthrough
case 111:
fallthrough
case 119:
fallthrough
case 123:
fallthrough
case 125:
fallthrough
case 126:
fallthrough
case 159:
fallthrough
case 175:
fallthrough
case 183:
fallthrough
case 187:
fallthrough
case 189:
fallthrough
case 190:
fallthrough
case 207:
fallthrough
case 215:
fallthrough
case 219:
fallthrough
case 221:
fallthrough
case 222:
fallthrough
case 231:
fallthrough
case 235:
fallthrough
case 237:
fallthrough
case 238:
fallthrough
case 243:
fallthrough
case 245:
fallthrough
case 246:
fallthrough
case 249:
fallthrough
case 250:
fallthrough
case 252:
counter = 6
case 127:
fallthrough
case 191:
fallthrough
case 223:
fallthrough
case 239:
fallthrough
case 247:
fallthrough
case 251:
fallthrough
case 253:
fallthrough
case 254:
counter = 7
case 255:
counter = 8
}
return counter
}
算法5
算法复杂度是O(1)，可以说是效率最高的了，perfect！即每次都只需运算一次就可以得出结果。 比如：计算出十进制数0-255的二进制数中1的个数都只需运算1次。
func tableCount(number int) int {
table := []int{
0, 1, 1, 2, 1, 2, 2, 3, 1, 2,
2, 3, 2, 3, 3, 4, 1, 2, 2, 3,
2, 3, 3, 4, 2, 3, 3, 4, 3, 4,
4, 5, 1, 2, 2, 3, 2, 3, 3, 4,
2, 3, 3, 4, 3, 4, 4, 5, 2, 3,
3, 4, 3, 4, 4, 5, 3, 4, 4, 5,
4, 5, 5, 6, 1, 2, 2, 3, 2, 3,
3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4,
4, 5, 4, 5, 5, 6, 2, 3, 3, 4,
3, 4, 4, 5, 3, 4, 4, 5, 4, 5,
5, 6, 3, 4, 4, 5, 4, 5, 5, 6,
4, 5, 5, 6, 5, 6, 6, 7, 1, 2,
2, 3, 2, 3, 3, 4, 2, 3, 3, 4,
3, 4, 4, 5, 2, 3, 3, 4, 3, 4,
4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4,
4, 5, 4, 5, 5, 6, 3, 4, 4, 5,
4, 5, 5, 6, 4, 5, 5, 6, 5, 6,
6, 7, 2, 3, 3, 4, 3, 4, 4, 5,
3, 4, 4, 5, 4, 5, 5, 6, 3, 4,
4, 5, 4, 5, 5, 6, 4, 5, 5, 6,
5, 6, 6, 7, 3, 4, 4, 5, 4, 5,
5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
4, 5, 5, 6, 5, 6, 6, 7, 5, 6,
6, 7, 6, 7, 7, 8,
}
return table[number]
}
总结
相比较算法1和算法2，算法3效率更高，算法4的计算效率是一个动态值，其计算效率依赖具体的数字。 相比之前3个算法效率有时候反而更低。但是该算法提供了一种很好的解决思路，即借助空间（内存）换取时间，将已知的值全部列出来，可以使用查找表实现算法，于是有算法5。算法5是通过查找表方式实现，通过空间（即内存）换取时间。由此可以看出，在不同的应用场景需选择不同算法实现，例如在内存很充裕且追求最高计算效率情况下，算法5是最适合的。但是在嵌入式开发过程中，内存受限且追求计算效率时，那么算法3是最适合的。