Update a DFS problem in LeetCode Daily.

Author: zh-plus

LeetCode/daily_challenge/CMakeLists.txt

@@ -1 +1 @@
-add_executable(daily_challenge p2021_3_24_AdvantageShuffle.cpp)
+add_executable(daily_challenge p_2021_3_26_PacificAtlanticWaterFlow.cpp)

LeetCode/daily_challenge/p_2021_3_26_PacificAtlanticWaterFlow.cpp

@@ -0,0 +1,141 @@
+//
+// Created by 10578 on 2021/3/26.
+//
+
+#include "iostream"
+#include "fstream"
+#include "sstream"
+#include "iterator"
+#include "utility"
+#include "vector"
+#include "unordered_set"
+#include "algorithm"
+#include "stack"
+#include "limits"
+#include "string"
+
+#define ALTER_IN(filename) std::fstream fs(filename); std::cin.rdbuf(fs.rdbuf());
+
+using pii = std::pair<int, int>;
+using namespace std;
+
+namespace std {
+    template<>
+    struct hash<pii> {
+        size_t operator()(const pii &p) const {
+            hash<int> hash_fn;
+            size_t seed = hash_fn(p.first);
+            return hash_fn(p.second) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
+        }
+    };
+}
+
+class Solution {
+public:
+    vector<vector<int>> pacificAtlantic(vector<vector<int>> &matrix) {
+        auto m = matrix.size(), n = matrix.front().size();
+        vector<vector<unordered_set<pii>>> adj(m, vector<unordered_set<pii>>(n));
+
+        unordered_set<pii> pac_set;
+        unordered_set<pii> atl_set;
+
+        for (auto i = 0; i < m; ++i) {
+            for (auto j = 0; j < n; ++j) {
+                int elem = matrix[i][j];
+
+                if (i - 1 > 0 && matrix[i - 1][j] >= elem) {
+                    adj[i][j].emplace(i - 1, j);
+                }
+
+                if (i + 1 < m && matrix[i + 1][j] >= elem) {
+                    adj[i][j].emplace(i + 1, j);
+                }
+
+                if (j - 1 > 0 && matrix[i][j - 1] >= elem) {
+                    adj[i][j].emplace(i, j - 1);
+                }
+
+                if (j + 1 < n && matrix[i][j + 1] >= elem) {
+                    adj[i][j].emplace(i, j + 1);
+                }
+            }
+        }
+
+        stack<pii> s;
+
+        // pacific ocean
+        for (auto i = 0; i < n; ++i) {
+            s.emplace(i, 0);
+        }
+        for (auto j = 1; j < m; ++j) {
+            s.emplace(0, j);
+        }
+        while (!s.empty()) {
+            auto curr = s.top();
+            s.pop();
+
+            pac_set.insert(curr);
+            for (auto &&x: adj[curr.first][curr.second]) {
+                if (pac_set.count(x) == 0) {
+                    s.push(x);
+                }
+            }
+        }
+
+        // atlantic ocean
+        for (auto i = 0; i < n; ++i) {
+            s.emplace(i, n - 1);
+        }
+        for (auto j = 1; j < m; ++j) {
+            s.emplace(m - 1, j);
+        }
+        while (!s.empty()) {
+            auto curr = s.top();
+            s.pop();
+
+            atl_set.insert(curr);
+            for (auto &&x: adj[curr.first][curr.second]) {
+                if (atl_set.count(x) == 0) {
+                    s.push(x);
+                }
+            }
+        }
+
+        auto result = vector<pii>();
+
+        copy_if(pac_set.begin(), pac_set.end(), back_inserter(result), [&](const pii &p) {
+            return atl_set.count(p) != 0;
+        });
+
+        auto final_result = vector<vector<int>>(result.size());
+        transform(result.begin(), result.end(), begin(final_result), [](pii x) {
+            return vector<int>{x.first, x.second};
+        });
+
+        return final_result;
+    }
+};
+
+int main() {
+    ALTER_IN("in.txt");
+
+    int m, n;
+    cin >> m >> n;
+    cin.ignore(numeric_limits<streamsize>::max(), '\n');
+
+    vector<vector<int>> matrix(m);
+    for (auto &x: matrix) {
+        string line;
+        getline(cin, line);
+        istringstream iss(line);
+
+        x = vector<int>{istream_iterator<int>(iss), istream_iterator<int>()};
+    }
+
+    Solution s;
+    for (auto p: s.pacificAtlantic(matrix)) {
+        cout << p[0] << " " << p[1] << endl;
+    }
+
+    return 0;
+}