@@ -92,7 +92,15 @@ The robot is initially standing on cell (1, 0), denoted by the -1.
9292
9393** 方法一:DFS 建图 + BFS 求最短路**
9494
95- 相似题目:[ 1810. 隐藏网格下的最小消耗路径] ( /solution/1800-1899/1810.Minimum%20Path%20Cost%20in%20a%20Hidden%20Grid/README.md )
95+ 我们不妨假设机器人从坐标 $(0, 0)$ 出发,那么我们可以通过 DFS,找到所有可达的坐标,记录在哈希表 $vis$ 中。另外,我们还需要记录终点的坐标 $target$。
96+ 97+ 如果找不到终点,那么直接返回 $-1$。否则,我们可以通过 BFS,求出最短路。
98+ 99+ 时间复杂度 $O(m \times n),ドル空间复杂度 $O(m \times n)$。其中 $m$ 和 $n$ 分别是网格的行数和列数。
100+ 101+ 相似题目:
102+ 103+ - [ 1810. 隐藏网格下的最小消耗路径] ( /solution/1800-1899/1810.Minimum%20Path%20Cost%20in%20a%20Hidden%20Grid/README.md )
96104
97105<!-- tabs:start -->
98106
@@ -118,31 +126,28 @@ The robot is initially standing on cell (1, 0), denoted by the -1.
118126
119127
120128class Solution (object ):
121- def findShortestPath (self master : ' GridMaster' int :
122- def dfs (i , j ):
123- nonlocal target
129+ def findShortestPath (self master : " GridMaster" int :
130+ def dfs (i : int , j : int ):
124131 if master.isTarget():
132+ nonlocal target
125133 target = (i, j)
126- for dir , ndir, a, b in dirs:
127- x, y = i + a, j + b
128- if master.canMove(dir ) and (x, y) not in s:
129- s.add((x, y))
130- master.move(dir )
134+ return
135+ for k, c in enumerate (s):
136+ x, y = i + dirs[k], j + dirs[k + 1 ]
137+ if master.canMove(c) and (x, y) not in vis:
138+ vis.add((x, y))
139+ master.move(c)
131140 dfs(x, y)
132- master.move(ndir )
141+ master.move(s[(k + 2 ) % 4 ] )
133142
143+ s = " URDL"
144+ dirs = (- 1 , 0 , 1 , 0 , - 1 )
134145 target = None
135- s = set ()
136- dirs = [
137- [' U' ' D' - 1 , 0 ],
138- [' D' ' U' 1 , 0 ],
139- [' L' ' R' 0 , - 1 ],
140- [' R' ' L' 0 , 1 ],
141- ]
146+ vis = set ()
142147 dfs(0 , 0 )
143148 if target is None :
144149 return - 1
145- s.remove ((0 , 0 ))
150+ vis.discard ((0 , 0 ))
146151 q = deque([(0 , 0 )])
147152 ans = - 1
148153 while q:
@@ -151,10 +156,10 @@ class Solution(object):
151156 i, j = q.popleft()
152157 if (i, j) == target:
153158 return ans
154- for _, _, a, b in dirs:
159+ for a, b in pairwise( dirs) :
155160 x, y = i + a, j + b
156- if (x, y) in s :
157- s .remove((x, y))
161+ if (x, y) in vis :
162+ vis .remove((x, y))
158163 q.append((x, y))
159164 return - 1
160165```
@@ -175,37 +180,31 @@ class Solution(object):
175180 */
176181
177182class Solution {
178- private static final char [] dir = {' U' ' R' ' D' ' L'
179- private static final char [] ndir = {' D' ' L' ' U' ' R'
180- private static final int [] dirs = {- 1 , 0 , 1 , 0 , - 1 };
181- private static final int N = 1010 ;
182- private Set<Integer > s;
183183 private int [] target;
184+ private GridMaster master;
185+ private final int n = 2010 ;
186+ private final String s = " URDL"
187+ private final int [] dirs = {- 1 , 0 , 1 , 0 , - 1 };
188+ private final Set<Integer > vis = new HashSet<> ();
184189
185190 public int findShortestPath (GridMaster master ) {
186- target = null ;
187- s = new HashSet<> ();
188- s. add(0 );
189- dfs(0 , 0 , master);
191+ this . master = master;
192+ dfs(0 , 0 );
190193 if (target == null ) {
191194 return - 1 ;
192195 }
193- s . remove(0 );
196+ vis . remove(0 );
194197 Deque<int[]> q = new ArrayDeque<> ();
195198 q. offer(new int [] {0 , 0 });
196- int ans = - 1 ;
197- while (! q. isEmpty()) {
198- ++ ans;
199- for (int n = q. size(); n > 0 ; -- n) {
200- int [] p = q. poll();
201- int i = p[0 ], j = p[1 ];
202- if (target[0 ] == i && target[1 ] == j) {
199+ for (int ans = 0 ; ! q. isEmpty(); ++ ans) {
200+ for (int m = q. size(); m > 0 ; -- m) {
201+ var p = q. poll();
202+ if (p[0 ] == target[0 ] && p[1 ] == target[1 ]) {
203203 return ans;
204204 }
205205 for (int k = 0 ; k < 4 ; ++ k) {
206- int x = i + dirs[k], y = j + dirs[k + 1 ];
207- if (s. contains(x * N + y)) {
208- s. remove(x * N + y);
206+ int x = p[0 ] + dirs[k], y = p[1 ] + dirs[k + 1 ];
207+ if (vis. remove(x * n + y)) {
209208 q. offer(new int [] {x, y});
210209 }
211210 }
@@ -214,24 +213,92 @@ class Solution {
214213 return - 1 ;
215214 }
216215
217- private void dfs (int i , int j , GridMaster master ) {
216+ private void dfs (int i , int j ) {
218217 if (master. isTarget()) {
219218 target = new int [] {i, j};
219+ return ;
220220 }
221221 for (int k = 0 ; k < 4 ; ++ k) {
222- char d = dir[k], nd = ndir[k];
223222 int x = i + dirs[k], y = j + dirs[k + 1 ];
224- if (master. canMove(d) && ! s. contains(x * N + y)) {
225- s. add(x * N + y);
226- master. move(d);
227- dfs(x, y, master);
228- master. move(nd);
223+ if (master. canMove(s. charAt(k)) && vis. add(x * n + y)) {
224+ master. move(s. charAt(k));
225+ dfs(x, y);
226+ master. move(s. charAt((k + 2 ) % 4 ));
229227 }
230228 }
231229 }
232230}
233231```
234232
233+ ### ** C++**
234+ 235+ ``` cpp
236+ /* *
237+ * // This is the GridMaster's API interface.
238+ * // You should not implement it, or speculate about its implementation
239+ * class GridMaster {
240+ * public:
241+ * bool canMove(char direction);
242+ * void move(char direction);
243+ * boolean isTarget();
244+ * };
245+ */
246+ 247+ class Solution {
248+ private:
249+ const int n = 2010;
250+ int dirs[ 5] = {-1, 0, 1, 0, -1};
251+ string s = "URDL";
252+ int target;
253+ unordered_set<int > vis;
254+ 255+ public:
256+ int findShortestPath(GridMaster& master) {
257+ target = n * n;
258+ vis.insert(0);
259+ dfs(0, 0, master);
260+ if (target == n * n) {
261+ return -1;
262+ }
263+ vis.erase(0);
264+ queue<pair<int, int>> q;
265+ q.emplace(0, 0);
266+ for (int ans = 0; q.size(); ++ans) {
267+ for (int m = q.size(); m; --m) {
268+ auto [ i, j] = q.front();
269+ q.pop();
270+ if (i * n + j == target) {
271+ return ans;
272+ }
273+ for (int k = 0; k < 4; ++k) {
274+ int x = i + dirs[ k] , y = j + dirs[ k + 1] ;
275+ if (vis.count(x * n + y)) {
276+ vis.erase(x * n + y);
277+ q.emplace(x, y);
278+ }
279+ }
280+ }
281+ }
282+ return -1;
283+ }
284+ 285+ void dfs(int i, int j, GridMaster& master) {
286+ if (master.isTarget()) {
287+ target = i * n + j;
288+ }
289+ for (int k = 0 ; k < 4 ; ++k) {
290+ int x = i + dirs[k], y = j + dirs[k + 1];
291+ if (master.canMove(s[k]) && !vis.count(x * n + y)) {
292+ vis.insert(x * n + y);
293+ master.move(s[k]);
294+ dfs (x, y, master);
295+ master.move(s[ (k + 2) % 4] );
296+ }
297+ }
298+ }
299+ };
300+ ```
301+ 235302### ** ...**
236303
237304```
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