Add Tutorial for LOJ-1175 Jane and the Frost Giants (en)

1175/en.md

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+# LOJ 1175 - Jane and the Frost Giants
+
+## Prerequisite
+[BFS (Bredth First Search)](https://cp-algorithms.com/graph/breadth-first-search.html)
+
+## Solution
+If we forget about the movable fire cells that grows every minute, then the problem reduces to calculating how many minutes it takes to reach a cell without going through an obstacle, which can be done easily with BFS alone.
+
+Similarly, with BFS, we can also compute the shortest time it takes
+fire to spread to a cell. Jane can only move through a cell if she
+reaches there before fire does.
+
+We can utilize this observation while traversing through the grid
+to find a route for Jane. If fire gets to an empty cell before
+she does, we can consider the cell an obstacle and move on.
+Otherwise, consider it empty and try to move through there.
+
+## Complexity
+- Time Complexity: O(T * R * C).
+- Memory Complexity: O(R * C).
+
+## Code
+
+### C++
+
+```cpp
+#include <bits/stdc++.h>
+
+using namespace std;
+
+const int INF = numeric_limits <int>:: max(); // Indicating Unreachable state
+
+int r, c;
+vector <string> grid;
+
+// An Efficient (and quite common) Way to Navigate Grid Problems: https://codeforces.com/blog/entry/78827
+const int dr[] = {-1, 0, 1, 0};
+const int dc[] = {0, 1, 0, -1};
+
+inline bool valid(int x, int y) {
+    // 0-based index grid
+    return x >= 0 && x < r && y >= 0 && y < c && grid[x][y] != '#';
+}
+
+struct Cell {
+    bool flag; // Indicating whther this cell is on fire or not
+    int x, y; // Row and column respectively 
+
+    Cell() {} // Default constrcutor
+    Cell(bool flag, int x, int y) : flag(flag), x(x), y(y) {}
+};
+
+int main() {
+
+    // For fast I/O
+    ios_base::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    int t;
+    cin >> t;
+
+    for(int ts = 1; ts <= t; ++ts) {
+        cin >> r >> c;
+
+        grid.resize(r);
+
+        pair <int, int> source;
+        queue <Cell> Q;
+        // dist[i][j] = Minimum minutes needed to move to cell[i][j] from starting cell
+        vector <vector <int>> dist(r, vector <int> (c, INF));
+        for(int i = 0; i < r; ++i) {
+            cin >> grid[i];
+            for(int j = 0; j < c; ++j) {
+                if (grid[i][j] == 'J') {
+                    source = {i, j};
+                }
+                else if (grid[i][j] == 'F') {
+                    // Turning into a movable obstacle
+                    grid[i][j] = '#';
+                    Q.push(Cell(true, i, j));
+                }
+            }
+        }
+
+        // All the fire cells have been added to the queue so that they always move to the adjacent cells first not, Jane
+        dist[source.first][source.second] = 0;
+        Q.push(Cell(false, source.first, source.second));
+
+        while (!Q.empty()) {
+            Cell u = Q.front(); 
+            Q.pop();
+
+            for(int i = 0; i < 4; ++i) {
+                int x = u.x + dr[i];
+                int y = u.y + dc[i];
+
+                // Checking for not an obstacle yet
+                if (valid(x, y) && dist[x][y] == INF) {
+                    if (u.flag) {
+                        // Now an obstacle that can move
+                        grid[x][y] = '#';
+                        Q.push(Cell(true, x, y));
+                    }
+                    else {
+                        // That's jane moving
+                        dist[x][y] = dist[u.x][u.y] + 1;
+                        Q.push(Cell(false, x, y));
+                    }
+                }
+            }
+        }
+
+        int best = INF;
+        // Topmost and downmost rows
+        for(int j = 0; j < c; ++j) {
+            best = min({best, dist[0][j], dist[r-1][j]});
+        }
+        // Leftmost and rightmost columns
+        for(int i = 0; i < r; ++i) {
+            best = min({best, dist[i][0], dist[i][c-1]});
+        }
+
+        cout << "Case " << ts << ": ";
+
+        if (best == INF) {
+            cout << "IMPOSSIBLE\n";
+        }
+        else {
+            // Adding 1 because of getting out of the maze completely
+            cout << best+1 << '\n'; 
+        }
+
+        grid.clear();
+    }
+
+    return 0;
+}
+```