题目链接：https://vjudge.net/problem/POJ-3278
题目大意
给定 N，K ，N 每次可以 +1，-1，*2，问 N 变到 K 最少要几次变化？
分析
一开始用 DP 死活做不出来，然后我去打了一周目魂2，再回来看看这道题。然后发现这是道 bfs，特水的那种。。。代码如下
1 #include <cmath>
2 #include <ctime>
3 #include <iostream>
4 #include <string>
5 #include <vector>
6 #include <cstdio>
7 #include <cstdlib>
8 #include <cstring>
9 #include <queue>
10 #include <map>
11 #include <set>
12 #include <algorithm>
13 #include <cctype>
14 #include <stack>
15 #include <deque>
16 #include <list>
17 #include <sstream>
18 using namespace std;
19
20 #define INIT() ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
21 #define Rep(i,n) for (int i = 0; i < (n); ++i)
22 #define For(i,s,t) for (int i = (s); i <= (t); ++i)
23 #define rFor(i,t,s) for (int i = (t); i >= (s); --i)
24 #define ForLL(i, s, t) for (LL i = LL(s); i <= LL(t); ++i)
25 #define rForLL(i, t, s) for (LL i = LL(t); i >= LL(s); --i)
26 #define foreach(i,c) for (__typeof(c.begin()) i = c.begin(); i != c.end(); ++i)
27 #define rforeach(i,c) for (__typeof(c.rbegin()) i = c.rbegin(); i != c.rend(); ++i)
28
29 #define pr(x) cout << #x << " = " << x << " "
30 #define prln(x) cout << #x << " = " << x << endl
31
32 #define LOWBIT(x) ((x)&(-x))
33
34 #define ALL(x) x.begin(),x.end()
35 #define INS(x) inserter(x,x.begin())
36 #define UNIQUE(x) x.erase(unique(x.begin(), x.end()), x.end())
37 #define REMOVE(x, c) x.erase(remove(x.begin(), x.end(), c), x.end()); // 删去 x 中所有 c
38 #define TOLOWER(x) transform(x.begin(), x.end(), x.begin(),::tolower);
39 #define TOUPPER(x) transform(x.begin(), x.end(), x.begin(),::toupper);
40
41 #define ms0(a) memset(a,0,sizeof(a))
42 #define msI(a) memset(a,inf,sizeof(a))
43 #define msM(a) memset(a,-1,sizeof(a))
44
45 #define MP make_pair
46 #define PB push_back
47 #define ft first
48 #define sd second
49
50 template<typename T1, typename T2>
51 istream &operator>>(istream &in, pair<T1, T2> &p) {
52 in >> p.first >> p.second;
53 return in;
54 }
55
56 template<typename T>
57 istream &operator>>(istream &in, vector<T> &v) {
58 for (auto &x: v)
59 in >> x;
60 return in;
61 }
62
63 template<typename T1, typename T2>
64 ostream &operator<<(ostream &out, const std::pair<T1, T2> &p) {
65 out << "[" << p.first << ", " << p.second << "]" << "\n";
66 return out;
67 }
68
69 inline int gc(){
70 static const int BUF = 1e7;
71 static char buf[BUF], *bg = buf + BUF, *ed = bg;
72
73 if(bg == ed) fread(bg = buf, 1, BUF, stdin);
74 return *bg++;
75 }
76
77 inline int ri(){
78 int x = 0, f = 1, c = gc();
79 for(; c<48||c>57; f = c=='-'?-1:f, c=gc());
80 for(; c>47&&c<58; x = x*10 + c - 48, c=gc());
81 return x*f;
82 }
83
84 template<class T>
85 inline string toString(T x) {
86 ostringstream sout;
87 sout << x;
88 return sout.str();
89 }
90
91 inline int toInt(string s) {
92 int v;
93 istringstream sin(s);
94 sin >> v;
95 return v;
96 }
97
98 //min <= aim <= max
99 template<typename T>
100 inline bool BETWEEN(const T aim, const T min, const T max) {
101 return min <= aim && aim <= max;
102 }
103
104 typedef long long LL;
105 typedef unsigned long long uLL;
106 typedef pair< double, double > PDD;
107 typedef pair< int, int > PII;
108 typedef pair< int, PII > PIPII;
109 typedef pair< string, int > PSI;
110 typedef pair< int, PSI > PIPSI;
111 typedef set< int > SI;
112 typedef set< PII > SPII;
113 typedef vector< int > VI;
114 typedef vector< double > VD;
115 typedef vector< VI > VVI;
116 typedef vector< SI > VSI;
117 typedef vector< PII > VPII;
118 typedef map< int, int > MII;
119 typedef map< int, string > MIS;
120 typedef map< int, PII > MIPII;
121 typedef map< PII, int > MPIII;
122 typedef map< string, int > MSI;
123 typedef map< string, string > MSS;
124 typedef map< PII, string > MPIIS;
125 typedef map< PII, PII > MPIIPII;
126 typedef multimap< int, int > MMII;
127 typedef multimap< string, int > MMSI;
128 //typedef unordered_map< int, int > uMII;
129 typedef pair< LL, LL > PLL;
130 typedef vector< LL > VL;
131 typedef vector< VL > VVL;
132 typedef priority_queue< int > PQIMax;
133 typedef priority_queue< int, VI, greater< int > > PQIMin;
134 const double EPS = 1e-8;
135 const LL inf = 0x3fffffff;
136 const LL infLL = 0x3fffffffffffffffLL;
137 const LL mod = 1e9 + 7;
138 const int maxN = 1e5 + 7;
139 const LL ONE = 1;
140 const LL evenBits = 0xaaaaaaaaaaaaaaaa;
141 const LL oddBits = 0x5555555555555555;
142
143 int N, K, ans;
144 queue< int > Q;
145 bool vis[maxN];
146
147 void bfs() {
148 int cnt = 1;
149 Q.push(N);
150
151 while(!Q.empty()) {
152 int tmp = Q.front(); Q.pop();
153 --cnt;
154 if(tmp == K) break;
155
156 if(BETWEEN(tmp, 0, maxN) && !vis[tmp]) {
157 vis[tmp] = true;
158 Q.push(tmp - 1);
159 Q.push(tmp + 1);
160 Q.push(tmp * 2);
161 }
162
163 if(cnt == 0) {
164 cnt = Q.size();
165 ++ans;
166 }
167 }
168 }
169
170 int main(){
171 //freopen("MyOutput.txt","w",stdout);
172 //freopen("input.txt","r",stdin);
173 INIT();
174 cin >> N >> K;
175 bfs();
176 cout << ans << endl;
177 return 0;
178 }
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