https://leetcode.com/problems/race-car/description/

1. BFS剪枝 0<=current position<=2*target。為什麽2*target有點不太明白

 1 class Solution {
 2 public:
 3     int dp[10002];
 4     struct Node{
 5         int pos;
 6         int speed;
 7         int step;
 8         Node(int p,int s,int ss){
 9             pos=p;
 
10             speed=s;
11             step=ss;
12         }
13     };
14     int racecar(int target) {
15         queue<Node> Q;
16         set<pair<int,int> > vis;
17         Q.push(Node(0,1,0));
18         vis.insert({0,1});
19         while(!Q.empty()){
20             Node q=Q.front();
 
21             Q.pop();
22             if(q.pos==target) return q.step;
23             if(vis.find({q.pos+q.speed,q.speed*2})==vis.end()&&q.pos+q.speed>=0&&q.pos+q.speed<=2*target){
24                 vis.insert({q.pos+q.speed,q.speed*2});
25                 Q.push(Node(q.pos+q.speed,q.speed*2
 
,q.step+1));
26             }
27             if(vis.find({q.pos,q.speed>0?-1:1})==vis.end()){
28                 vis.insert({q.pos,q.speed>0?-1:1});
29                 Q.push(Node(q.pos,q.speed>0?-1:1,q.step+1));
30             }
31         }
32         return -1;
33     }
34 };

2. Dijkstra，官方題解，沒太看懂

3. DP

 1 class Solution {
 2 public:
 3     int dp[10002];
 4     int racecar(int target) {
 5         if(dp[target]!=0) return dp[target];
 6         int bound;
 7         for(int i=0;i<32;i++){
 8             if((1<<i)-1==target) return dp[target]=i;
 9             if((1<<i)-1>target){
10                 bound=i;
11                 break;
12             }
13         }
14         dp[target]=bound+1+racecar((1<<bound)-1-target);
15         for(int i=0;i<bound-1;i++){
16             dp[target]=min(dp[target],bound-1+1+i+1+racecar(target-(((1<<(bound-1))-1)-((1<<i)-1))));
17         }
18         return dp[target];
19     }
20 };

【leetcode最短路】818. Race Car