[CF792D] Paths in a Complete Binary Tree (规律, 位运算, lowbit)

题目链接：http://codeforces.com/problemset/problem/792/D

画出树，找找规律，画图就好了。不算麻烦。

往下走的时候特判是不是叶子，往上走的时候特判是不是根。其余时候按照规律转移就是。

感觉可以推广到建树上，可以缩小常数是极好的。

#include <bits/stdc++.h>
using namespace std;

typedef long long LL;
LL lowbit(LL x) { return x & (-x); }
const int maxn = 100010;
LL n, u;
int q;
char s[maxn];

signed main() {
    // freopen("in", "r", stdin);
    LL x, y, rt;
    while(~scanf("%lld%d",&n,&q)) {
        rt = 1;
        while((1LL << rt) < n) rt++;
        rt = 1LL << rt; rt >>= 1;
        while(q--) {
            scanf("%lld%s",&u,s);
            bool ok = 1;
            for(int i = 0; s[i]; i++) {
                x = u; y = lowbit(x);
                if(s[i] == 'L') {
                    if(u & 1) continue;
                    x ^= y; x |= (y >> 1);
                    if(x >= 1) u = x;
                }
                else if(s[i] == 'R') {
                    if(u & 1) continue;
                    x |= (y >> 1);
                    if(x <= n) u = x;
                }
                else {
                    if(x == (x ^ y)) {
                        if(u != rt) u = (x ^ y);
                    }
                    else {
                        if(u != rt) u = (x ^ y) | (y << 1);
                    }
                }
            }
            printf("%lld
", u);
        }
    }
    return 0;
}