OJ 必须要了解的算法（如果要和别人比时间的话）

1. 数据量巨大的时候 std::cin, std::cout TLE (超时)

static bool speedUP = [](){std::ios::sync_with_stdio(false); cin.tie(nullptr); return true}();

// 静态变量先于 main 函数初始化，以实现直接调用 lambda 函数

// std::ios::sync_with_stdio (false); 用以关闭 同步功能，不写缓存，不 flush

// cin.tie (nullptr); 用于绑定输入流，nullptr 为当前输入流

// 写了一个 lambda 函数并调用了

https://byvoid.com/zhs/blog/fast-readfile/

https://zhuanlan.zhihu.com/p/35652783

https://www.jianshu.com/p/fa8ad995d300

2. 快速整数平方根（牛顿迭代打表）（std::sqrt 的 4 倍）

inline int mysqrt(int n) {

    static int table[256] = {

        0,    16,  22,  27,  32,  35,  39,  42,  45,  48,  50,  53,  55,  57,

        59,   61,  64,  65,  67,  69,  71,  73,  75,  76,  78,  80,  81,  83,

        84,   86,  87,  89,  90,  91,  93,  94,  96,  97,  98,  99, 101, 102,

        103, 104, 106, 107, 108, 109, 110, 112, 113, 114, 115, 116, 117, 118,

        119, 120, 121, 122, 123, 124, 125, 126, 128, 128, 129, 130, 131, 132,

        133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 144, 145,

        146, 147, 148, 149, 150, 150, 151, 152, 153, 154, 155, 155, 156, 157,

        158, 159, 160, 160, 161, 162, 163, 163, 164, 165, 166, 167, 167, 168,

        169, 170, 170, 171, 172, 173, 173, 174, 175, 176, 176, 177, 178, 178,

        179, 180, 181, 181, 182, 183, 183, 184, 185, 185, 186, 187, 187, 188,

        189, 189, 190, 191, 192, 192, 193, 193, 194, 195, 195, 196, 197, 197,

        198, 199, 199, 200, 201, 201, 202, 203, 203, 204, 204, 205, 206, 206,

        207, 208, 208, 209, 209, 210, 211, 211, 212, 212, 213, 214, 214, 215,

        215, 216, 217, 217, 218, 218, 219, 219, 220, 221, 221, 222, 222, 223,

        224, 224, 225, 225, 226, 226, 227, 227, 228, 229, 229, 230, 230, 231,

        231, 232, 232, 233, 234, 234, 235, 235, 236, 236, 237, 237, 238, 238,

        239, 240, 240, 241, 241, 242, 242, 243, 243, 244, 244, 245, 245, 246,

        246, 247, 247, 248, 248, 249, 249, 250, 250, 251, 251, 252, 252, 253,

        253, 254, 254, 255

     };

    int xn;

    if (n >= 0x7FFEA810) return 0xB504;

    if (n >= 0x10000) {

        if (n >= 0x1000000) {

            if (n >= 0x10000000) {

                if (n >= 0x40000000) {

                     xn = table[n>> 24] << 8;

                 } else {

                     xn = table[n>> 22] << 7;

                 }

             } else {

                if (n>= 0x4000000) {

                     xn = table[n>> 20] << 6;

                 } else {

                     xn = table[n>> 18] << 5;

                 }

             }

             xn = (xn + 1 + (n/ xn)) >> 1;

             xn = (xn + 1 + (n/ xn)) >> 1;

            return ((xn * xn) > n) ? --xn : xn;

         } else {

            if (n>= 0x100000) {

                if (n>= 0x400000) {

                     xn = table[n>> 16] << 4;

                 } else {

                     xn = table[n>> 14] << 3;

                 }

             } else {

                if (n>= 0x40000) {

                     xn = table[n>> 12] << 2;

                 } else {

                     xn = table[n>> 10] << 1;

                 }

             }

             xn = (xn + 1 + (n/ xn)) >> 1;

            return ((xn * xn) > n) ? --xn : xn;

         }

     } else {

        if (n>= 0x100) {

            if (n>= 0x1000) {

                if (n>= 0x4000) {

                     xn = (table[n>> 8]) + 1;

                 } else {

                     xn = (table[n>> 6] >> 1) + 1;

                 }

             } else {

                if (n>= 0x400) {

                     xn = (table[n>> 4] >> 2) + 1;

                 } else {

                     xn = (table[n>> 2] >> 3) + 1;

                 }

             }

            return ((xn * xn) > n) ? --xn : xn;

         } else {

            if (n>= 0) {

                return table[n] >> 4;

             }

         }

     }

    return -1;

}

https://blog.csdn.net/hunterlew/article/details/45341253

https://blog.csdn.net/xtlisk/article/details/51249371

https://blog.csdn.net/wanchuanhua/article/details/5996708

http://bbs.mydigit.cn/simple/?t2329623.html

https://www.cnblogs.com/signal/p/3818332.html

https://www.cnblogs.com/atyuwen/archive/2009/11/09/1598942.html