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LeetCode/Algorithms/Hard/BusRoutes.cpp

@@ -0,0 +1,63 @@
+class Solution {
+public:
+    int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
+        int n = routes.size();
+        int level = 0;  
+
+        unordered_map<int, vector<int>> m;   // store - bus stop no. -> bus numbers list 
+
+        for(int i=0; i<n; i++) {
+            for(int j=0; j<routes[i].size(); j++) {
+                int busStopNo = routes[i][j]; 
+                int busNo = i; 
+                m[busStopNo].push_back(busNo);
+            }
+        }
+
+        // bfs 
+        queue<int> queue;
+        unordered_set<int> busStopVisited; 
+        unordered_set<int> busVisited;  
+
+        queue.push(source);
+        busStopVisited.insert(source); 
+
+        while(!queue.empty()) {
+            int size = queue.size(); 
+            while(size--) {
+                int rem = queue.front(); 
+                queue.pop(); 
+
+                if(rem == target) {
+                    return level; 
+                }
+
+                vector<int> buses = m[rem]; 
+                for(int bus: buses) {
+                    if(busVisited.find(bus) != busVisited.end()) {
+                        continue; 
+                    }
+
+                    vector<int> arr = routes[bus]; 
+
+                    for(int busStop: arr) {
+                        if(busStopVisited.find(busStop) != busStopVisited.end()) {
+                            continue; 
+                        } 
+
+                        queue.push(busStop); 
+                        busStopVisited.insert(busStop); 
+                    }
+                    busVisited.insert(bus); 
+                }
+            }
+            level += 1; 
+        }
+
+        return -1; 
+
+    }
+};
+
+// Time Complexity - O(N) 
+// Space Complexity - O(N), since we're using a map to keep track of bus and busStop

LeetCode/Algorithms/Hard/CherryPickup2.cpp

@@ -0,0 +1,124 @@
+class Solution {
+public:
+    int cherryPickup(vector<vector<int>>& grid) {
+        int r = grid.size(); 
+        int c = grid[0].size(); 
+        vector<vector<vector<int>>> dp(r, vector<vector<int>>(c, vector<int>(c, -1)));
+        return func(0, 0, c-1, r, c, grid, dp);
+    }
+
+    int func(int i, int j1, int j2, int r, int c, vector<vector<int>> &grid, vector<vector<vector<int>>> &dp) {
+        if(i < 0 || j1 < 0 || j2 < 0 || j1 >= c || j2 >= c) {
+            return -1e5; 
+        }
+
+        if(dp[i][j1][j2] != -1) {
+            return dp[i][j1][j2]; 
+        }
+
+        if(i == r -1 ) {
+            if(j1 == j2) {
+                return grid[i][j1]; 
+            } else {
+                return grid[i][j1] + grid[i][j2]; 
+            }
+        }
+
+        // explore all paths of alice and bob simultaneously 
+        int maxi = -1e5; 
+        for(int dj1 = -1; dj1 <= +1; dj1++) {
+            for(int dj2 = -1; dj2 <= +1; dj2++) {
+                int value = 0; 
+                if(j1 == j2) {
+                    value = grid[i][j1];
+                } else {
+                    value = grid[i][j1] + grid[i][j2]; 
+                }
+
+                value += func(i+1, j1 + dj1, j2 + dj2, r, c, grid, dp); 
+                maxi = max(maxi, value); 
+            }
+        }
+        return dp[i][j1][j2] = maxi; 
+    }
+};
+
+
+// Rules 
+// 1. Express everything in terms (i1, j1) && (i2, j2); 
+// 2. Explore all the paths (i+1, j-1), (i + 1, j), (i + 1, j + 1)
+// 3. Give maximum sum 
+
+// Fixed starting point     variable ending point
+// (0, 0)    (0, m-1)        (set any column in the last row)
+
+// (all paths by Alice) + (all paths by Bob)
+// (recursion)              (recursion)
+
+// f(i1, j1, i2, j2)
+// f(0, 0, 0, m-1);
+//   alice   bob 
+
+// base case 
+// 1. Destination base case 
+// 2. Out of bounds case 
+
+// f(i, j1, j2)  // i1 == i2;
+// {
+//     if(i1 < 0 || j1 >= m || j2 < 0 || j2 >= m) {
+//         return -1e8;  // not INT_MIN (since it can go out of bounds)
+//     }
+
+//     if(dp[i][j1][j2] != -1) {
+//         return dp[i][j1][j2]; 
+//     }
+
+//     if(i == n-1) {
+//         if(j1 == j2) {
+//             return a[i][j1]; 
+//         } else {
+//             return a[i][j1] + a[i][j2]; 
+//         }
+//     }
+
+// }
+
+// 3 * 3 = 9 combos of paths 
+// for each movement of alice, there are 3 moves for Bob 
+
+// dj[] = {-1, 0, +1};
+// fun(del1() -> -1 -> +1) 
+// fun(del2() -> -1 -> +1)
+
+// Explore all paths Alice & Bob can go together 
+
+// func(){
+// for(dj1 -> -1 -> +1) {
+//     for(dj2 -> -1 -> +1) {
+//         if(j1 == j2) {
+//             maxi = max(maxi, a[i][j1] + f(i + 1, j1 + dj1, j2 + dj2)); 
+//         } else {
+//             maxi = max(maxi, a[i][j1] + a[i][j2] + f(i + 1, j1 + dj1, j2 + dj2)); 
+//         }
+//     }
+// }
+// dp[i][j1][j2] = maxi; 
+// return maxi; 
+// }
+
+// Recursion - TC -> (3^n X 3^n), exponential 
+//             SC -> O(N), auxillary stack space 
+
+// Optimize - find overlapping sub problems 
+// Memoization 
+
+// i - N 
+// j1 - M 
+// j2 - M 
+
+// dp[i][j1][j2] = maxi; 
+
+// Memoization - TC - O(N * M * M) * 9 
+// Space Complexity - O(N * M * M) * O(N), O(N) is the Auxillary stack space 
+
+

LeetCode/Algorithms/Hard/FindTheMaximumSumOfNodeValues.cpp

@@ -0,0 +1,45 @@
+class Solution {
+public:
+    long long maximumValueSum(vector<int>& nums, int k, vector<vector<int>>& edges) {
+        // When would XOR increase a value? 
+        // What if it increases one but decreases other? 
+        // One num can be XOR'd multiple times 
+        // Does a traversal make sense? 
+
+        // XOR is not a blackbox, n ^ k ^ k = n 
+        // Thus we can (and must) XOR any two nodes at a time 
+        // Greedy, sort by XOR delta
+
+        long long result = 0; 
+        vector<long long> delta; 
+        for(long long i=0; i<nums.size(); i++) {
+            result += nums[i]; 
+            delta.push_back((nums[i] ^ k) - nums[i]); 
+        }
+
+        sort(delta.begin(), delta.end(), greater<>()); 
+        for(long long i=0; i<delta.size(); i+= 2) {
+            if(i == delta.size() - 1) {
+                break; 
+            }
+            long long path_delta = delta[i] + delta[i + 1]; 
+            if(path_delta <= 0) {
+                break; 
+            }
+            result += path_delta; 
+        }
+
+        return result; 
+    }
+};
+
+// Tree is a connected graph that doesn't have cycles in it
+
+// n ^ k ^ k = n 
+
+// either you leave the node as it is or XOR nodes twice
+
+// maximize the total sum of values
+
+// Time Complexity - O(NlogN) 
+// Space Complexity - O(N), extra space