问题描述
我刚刚学习了BFS算法，并且尝试在Open the Lock
上应用BFS算法解决leetcode问题。我的算法适用于某些用例，而对于其他用例则输出错误答案。谁能帮助我了解我所缺少的东西吗？
预先感谢
class Solution {
Queue<String> queue = new LinkedList<String>();
HashSet<String> deads = new HashSet<String>();
public int openLock(String[] deadends,String target) {
for(int i = 0; i < deadends.length; i++){
deads.add(deadends[i]);
}
if(deads.contains("0000"))return -1;
int level = bfs("0000",target);
return level;
}
public int bfs(String start,String target){
int level = 0;
queue.add(start); // add the start to the queue
deads.add(start);
while(!queue.isEmpty()){
int groupSize = queue.size();
while(groupSize >0){
String current = queue.poll();
if(current.equals(target)) return level;
for(int i = 0; i < current.length(); i++){
char c = current.charat(i);
char temp = c;
if( c == '9'){
c = '0';
temp = c;
}else{
c++;
}
String upString = current.substring(0,i) + c + current.substring(i + 1);
if(!deads.contains(upString)){
queue.add(upString);
deads.add(upString);
}
c = temp;
if( c == '0'){
c = '9';
}
else{
c--;
}
String downString = current.substring(0,i) + c + current.substring(i + 1);
if(!deads.contains(downString)){
queue.add(downString);
deads.add(downString);
}
}
groupSize = groupSize - 1;
}
level = level + 1;
}
return -1;
}
}
解决方法
不太确定您的算法可能有什么问题，对我来说还不错，可能是一些需要修复的小问题。
几乎相同的方法，除了我们将使用三个Set来解决Java中的问题：
public final class Solution {
public static final int openLock(
final String[] deadends,final String target
) {
Set<String> startSet = new HashSet<>();
Set<String> endSet = new HashSet<>();
Set<String> deadSet = new HashSet<>(Arrays.asList(deadends));
startSet.add("0000");
endSet.add(target);
int minTurns = 0;
Set<String> tempSet;
while (!startSet.isEmpty() && !endSet.isEmpty()) {
if (startSet.size() > endSet.size()) {
tempSet = startSet;
startSet = endSet;
endSet = tempSet;
}
tempSet = new HashSet<>();
for (String start : startSet) {
if (endSet.contains(start)) {
return minTurns;
}
if (deadSet.contains(start)) {
continue;
}
deadSet.add(start);
StringBuilder startSB = new StringBuilder(start);
for (int i = 0; i < 4; i++) {
final char character = startSB.charAt(i);
String s1 = startSB.substring(0,i) + (character == '9' ? 0 : character - '0' + 1) + startSB.substring(i + 1);
String s2 = startSB.substring(0,i) + (character == '0' ? 9 : character - '0' - 1) + startSB.substring(i + 1);
if (!deadSet.contains(s1)) {
tempSet.add(s1);
}
if (!deadSet.contains(s2)) {
tempSet.add(s2);
}
}
}
minTurns++;
startSet = tempSet;
}
return -1;
}
}
好！这是LeetCode的BFS解决方案，您可以根据该解决方案进行解决：
class Solution {
public int openLock(String[] deadends,String target) {
Set<String> dead = new HashSet();
for (String d: deadends) dead.add(d);
Queue<String> queue = new LinkedList();
queue.offer("0000");
queue.offer(null);
Set<String> seen = new HashSet();
seen.add("0000");
int depth = 0;
while (!queue.isEmpty()) {
String node = queue.poll();
if (node == null) {
depth++;
if (queue.peek() != null)
queue.offer(null);
} else if (node.equals(target)) {
return depth;
} else if (!dead.contains(node)) {
for (int i = 0; i < 4; ++i) {
for (int d = -1; d <= 1; d += 2) {
int y = ((node.charAt(i) - '0') + d + 10) % 10;
String nei = node.substring(0,i) + ("" + y) + node.substring(i+1);
if (!seen.contains(nei)) {
seen.add(nei);
queue.offer(nei);
}
}
}
}
}
return -1;
}
}
似乎他们为这个问题添加了新的测试用例。
参考文献
- 有关其他详细信息，请参见Discussion Board，在这里您可以找到许多具有各种languages且已被广泛接受的解决方案，包括低复杂度算法和渐近runtime / {{ 3}}分析memory，1 。
class Solution {
Queue<String> queue = new LinkedList<String>();
HashSet<String> deads = new HashSet<String>();
public int openLock(String[] deadends,String target) {
for(int i = 0; i < deadends.length; i++){
deads.add(deadends[i]);
}
if(deads.contains("0000"))return -1;
int level = bfs("0000",target);
return level;
}
public int bfs(String start,String target){
int level = 0;
queue.add(start); // add the start to the queue
deads.add(start);
while(!queue.isEmpty()){
int groupSize = queue.size();
while(groupSize >0){
String current = queue.poll();
if(current.equals(target)) return level;
for(int i = 0; i < current.length(); i++){
char c = current.charAt(i);
char temp = c;
if( c == '9'){
c = '0';
temp = c; // THIS LINE IS THE ISSUE. REMOVING IT HELPED!!!!
}else{
c++;
}
String upString = current.substring(0,i) + c + current.substring(i + 1);
if(!deads.contains(upString)){
queue.add(upString);
deads.add(upString);
}
c = temp;
if( c == '0'){
c = '9';
}
else{
c--;
}
String downString = current.substring(0,i) + c + current.substring(i + 1);
if(!deads.contains(downString)){
queue.add(downString);
deads.add(downString);
}
}
groupSize = groupSize - 1;
}
level = level + 1;
}
return -1;
}
}