LeetCode BFS(下)
这里是LeetCode BFS类型后半部分题目：
#1.Surrounded Regions ref
Given a 2D board containing ‘X’ and ‘O’, capture all regions surrounded by ‘X’.
A region is captured by flipping all ‘O’s into ‘X’s in that surrounded region. For example,
X X X X
X O O X
X X O X
X O X X
After running your function, the board should be:
X X X X
X X X X
X X X X
X O X X
题目大意是将X包围的O全部改为X。 ###思路一 对矩阵中的任一位置的O，判断它能否找到一个全O路径(上下左右移动)，通向外界。OK, 这里DFS和BFS算法均可，下面是一个BFS代码
class Solution {
int m;
int n;
bool isClosure(vector<vector<char>> &board, int x, int y)
{
vector<vector<bool>> visited(m, vector<bool>(n, false));
queue<pair<int, int>> q;
pair<int, int> pos = { x, y };
q.push(pos);
visited[x][y] = true;
while (!q.empty())
{
pos = q.front();
q.pop();
x = pos.first;
y = pos.second;
if (x == 0 || x == m - 1 || y == 0 || y == n - 1)
{
return false;
}
//up, down, left, right
if (x > 0 && board[x - 1][y] == 'O' && !visited[x - 1][y])
{
q.push(pair<int, int>(x - 1, y));
visited[x - 1][y] = true;
}
if (x < m - 1 && board[x + 1][y] == 'O' && !visited[x + 1][y])
{
q.push(pair<int, int>(x + 1, y));
visited[x + 1][y] = true;
}
if (y > 0 && board[x][y - 1] == 'O' && !visited[x][y - 1])
{
q.push(pair<int, int>(x, y - 1));
visited[x][y - 1] = true;
}
if (y < n - 1 && board[x][y + 1] == 'O' && !visited[x][y + 1])
{
q.push(pair<int, int>(x, y + 1));
visited[x][y + 1] = true;
}
}
return true;
}
public:
//判断 O 能不能找到一路径出去
//如果不能出去就置 X
void solve(vector<vector<char>> &board)
{
m = board.size();
if (m < 1)
{
return;
}
n = board[0].size();
for (int i = 1; i < m - 1; i++)
{
for (int j = 1; j < n - 1; j++)
{
if (board[i][j] == 'O' && isClosure(board, i, j))
{
board[i][j] = 'X';
}
}
}
}
}; 但是当矩阵规模200x200以上时会超时Time Limit Exceeded。显然有更好的算法。
###思路二
上述问题之所以超时，是有太多的重复搜索，我考虑过在搜索完成一次，如果没有找到路径，就将访问过的位置全部置X，可是当矩阵规模达到250x250，还是超时。
从外向里搜索，对于O的位置，其周围的O位置保持，其它未搜索到的O就反转为X，这样就避免了重复搜索。
class Solution {
public:
//from outside to inside
void solve(vector<vector<char>> &board)
{
int m = board.size();
if (m < 1)
{
return;
}
int n = board[0].size();
typedef pair<int, int> POS;
queue<POS> q;
POS pos;
vector<vector<bool>> xo(m, vector<bool>(n, true));//true for x
//left and right
for (int i = 0; i < m; i++)
{
if (board[i][0] == 'O')
{
q.push(POS(i, 0));
xo[i][0] = false;
}
if (board[i][n - 1] == 'O')
{
q.push(POS(i, n - 1));
xo[i][n - 1] = false;
}
}
//up and down
for (int i = 0; i < n; i++)
{
if (board[0][i] == 'O')
{
q.push(POS(0, i));
xo[0][i] = false;
}
if (board[m - 1][i] == 'O')
{
q.push(POS(m - 1, i));
xo[m - 1][i] = false;
}
}
while (!q.empty())
{
pos = q.front();
q.pop();
if (pos.first > 0 && board[pos.first - 1][pos.second] == 'O' && xo[pos.first - 1][pos.second])
{
xo[pos.first - 1][pos.second] = false;
q.push(POS(pos.first - 1, pos.second));
}
if (pos.first < m - 1 && board[pos.first + 1][pos.second] == 'O' && xo[pos.first + 1][pos.second])
{
xo[pos.first + 1][pos.second] = false;
q.push(POS(pos.first + 1, pos.second));
}
if (pos.second > 0 && board[pos.first][pos.second - 1] == 'O' && xo[pos.first][pos.second - 1])
{
xo[pos.first][pos.second - 1] = false;
q.push(POS(pos.first, pos.second - 1));
}
if (pos.second < n - 1 && board[pos.first][pos.second + 1] == 'O' && xo[pos.first][pos.second + 1])
{
xo[pos.first][pos.second + 1] = false;
q.push(POS(pos.first, pos.second + 1));
}
}
for (int i = 0; i < m; i++)
{
for (int j = 0; j < n; j++)
{
board[i][j] = xo[i][j] ? 'X' : 'O';
}
}
}
}; #2.Word Ladder ref
Given two words (start and end), and a dictionary, find the length of shortest transformation sequence from start to end, such that:
- Only one letter can be changed at a time
- Each intermediate word must exist in the dictionary
For example,
Given:
start = "hit"
end = "cog"
dict = ["hot","dot","dog","lot","log"]
As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",
return its length 5.
Note:
- Return 0 if there is no such transformation sequence.
- All words have the same length.
- All words contain only lowercase alphabetic characters.
这道题目我又是先想了一个简单而超时的解法，还是分开说一下吧 ###思路一
- 将ladder当前的单词记为word，将word可能变化成的单词记为chword，word初始为start，chword与word只相差一个字母;
- 遍历word所有字符位置，依次改变一个位置，对每一个位置有25种变化可能;
- 如果chword等于end，直接返回;
- 如果chword存在于字典dict中，则加入队列q;
- 队列中的每一个单词有一个ladder length，表示从start单词变化到此的ladder长度，初始length=1;
int ladderLength(string start, string end, unordered_set<string> &dict)
{
if (end.compare(start) == 0)
{
return 1;
}
int length = 0;
string word;
string nword;
typedef pair<string, int> PAIR;
queue<PAIR > q;
q.push(PAIR(start, 1));
while (!q.empty())
{
word = q.front().first;
length = q.front().second;
q.pop();
//change position in start word
for (int i = 0; i < start.size(); i++)
{
//26 alpha
for (int j = 0; j < 26; j++)
{
if (word[i] == 'a' + j)
{
continue;
}
nword = word;
nword.replace(i, 1, 1, char('a' + j));
//cout << nword << endl;
if (end.compare(nword) == 0)
{
return length + 1;
}
if (dict.find(nword) != dict.end())
{
q.push(PAIR(nword, length + 1));
}
}
}
}
return 0;
}可惜又超时了。
###思路二 分析一下超时的原因，看上面代码，有两个for循环，第一个遍历单词的所有位置，不可或缺！第二个for循环遍历所有字母，有很多无用功(根本不在dict中)，看起来可以优化。
- compare()函数，没必要每次变化出一个单词就比较，可以在第一个for循环前面加上一个判断，看看word和end之间是不是只差一个字母。
- hot -> dot 之后，在搜索dot时，第一个字母不应该再变化了，因为dot变换首字母的情况和hot变换首字母的情况重复了，而思路一明显没有考虑这种情况，可能会造成无穷循环。解决办法是
增加一个unordered_set
used; 用于在插入队列时判断这个单词是否已经入过队列；或者直接在dict中删除。
int diffnum(string s1, string s2)
{
int n = 0;
for (int i = 0; i < s1.size(); i++)
{
if (s1[i] != s2[i])
{
n++;
}
}
return n;
}
int ladderLength(string start, string end, unordered_set<string> &dict)
{
if (end.compare(start) == 0)
{
return 1;
}
bool cmpflag = false;
int length = 0;
string word;
string nword;
typedef pair<string, int> PAIR;
queue<PAIR > q;
unordered_set<string> used;
q.push(PAIR(start, 1));
while (!q.empty())
{
word = q.front().first;
length = q.front().second;
q.pop();
//first judge the diff letter num
cmpflag = diffnum(word, end) == 1 ? true : false;
//change position in start word
for (int i = 0; i < start.size(); i++)
{
for (char c = 'a'; c <= 'z'; c++)
{
if (word[i] == c)
{
continue;
}
nword = word;
nword[i] = c;
//cout << nword << endl;
if (cmpflag && end.compare(nword) == 0)
{
return length + 1;
}
if (dict.find(nword) != dict.end() && (used.find(nword) == used.end()))
{
q.push(PAIR(nword, length + 1));
used.insert(nword);
}
}
}
}
return 0;
}#3.Word Ladder II ref
Given two words (start and end), and a dictionary, find all shortest transformation sequence(s) from start to end, such that:
- Only one letter can be changed at a time
- Each intermediate word must exist in the dictionary
For example,
Given:
start = “hit”
end = “cog”
dict = [“hot”,”dot”,”dog”,”lot”,”log”]
Return
[
[“hit”,”hot”,”dot”,”dog”,”cog”],
[“hit”,”hot”,”lot”,”log”,”cog”]
]
Note:
- All words have the same length.
- All words contain only lowercase alphabetic characters.
下面是参考一个大牛的代码，其主要思路是把每个单词的前驱记录下来，用vector
class Solution {
public:
vector<vector<string> > findLadders(string start, string end, unordered_set<string> &dict) {
pathes.clear();
dict.insert(start);
dict.insert(end);
vector<string> prev;
unordered_map<string, vector<string> > traces;
for (unordered_set<string>::const_iterator citr = dict.begin();citr != dict.end(); citr++) {
traces[*citr] = prev;
}
vector<unordered_set<string> > layers(2);
int cur = 0;
int pre = 1;
layers[cur].insert(start);
while (true) {
cur = !cur;
pre = !pre;
for (unordered_set<string>::const_iterator citr = layers[pre].begin();
citr != layers[pre].end(); citr++)
dict.erase(*citr);
layers[cur].clear();
for (unordered_set<string>::const_iterator citr = layers[pre].begin();
citr != layers[pre].end(); citr++) {
for (int n = 0; n<(*citr).size(); n++) {
string word = *citr;
int stop = word[n] - 'a';
for (int i = (stop + 1) % 26; i != stop; i = (i + 1) % 26) {
word[n] = 'a' + i;
if (dict.find(word) != dict.end()) {
traces[word].push_back(*citr);
layers[cur].insert(word);
}
}
}
}
if (layers[cur].size() == 0)
return pathes;
if (layers[cur].count(end))
break;
}
vector<string> path;
buildPath(traces, path, end);
return pathes;
}
private:
void buildPath(unordered_map<string, vector<string> > &traces,
vector<string> &path, const string &word) {
if (traces[word].size() == 0) {
path.push_back(word);
vector<string> curPath = path;
reverse(curPath.begin(), curPath.end());
pathes.push_back(curPath);
path.pop_back();
return;
}
const vector<string> &prevs = traces[word];
path.push_back(word);
for (vector<string>::const_iterator citr = prevs.begin();
citr != prevs.end(); citr++) {
buildPath(traces, path, *citr);
}
path.pop_back();
}
vector<vector<string> > pathes;
};
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