Dec1: Backtrack, Greedy train

daily/Dec1.cc

@@ -0,0 +1,114 @@
+#include <functional>
+#include <vector>
+
+using namespace std;
+
+class Solution {
+ public:
+  /**
+   * @brief LC weekly: Bitwise ops
+   * Bitwise OR(|) and Shifts(>>): fills all bits to the right of
+   * the highest set bit with ones.
+   * @param n
+   * @return int
+   */
+  int smallestNumber(int n) {
+    int num = n;
+    num |= num >> 1;
+    num |= num >> 2;
+    num |= num >> 4;
+    num |= num >> 8;
+    num |= num >> 16;
+
+    return num;
+  }
+
+  /**
+   * @brief LC51: NQueen
+   * Backtrack
+   *
+   * @param n
+   * @return vector<vector<string>>
+   */
+  vector<vector<string>> solveNQueens(int n) {
+    vector<vector<string>> ans;
+    vector<int> col(n), on_path(n), diag1(n * 2 - 1), diag2(n * 2 - 1);
+    function<void(int)> dfs = [&](int r) {
+      // traverse the last row, end
+      if (r == n) {
+        vector<string> board(n);
+        for (int i = 0; i < n; i++) {
+          board[i] = string(col[i], '.') + 'Q' + string(n - 1 - col[i], '.');
+        }
+        ans.emplace_back(board);
+        return;
+      }
+
+      // just traverse every row & col
+      for (int c = 0; c < n; c++) {
+        int rc = r - c + n - 1;
+        if (!on_path[c] && !diag1[r + c] && !diag2[rc]) {
+          col[r] = c;
+          // place at pos
+          on_path[c] = diag1[r + c] = diag2[rc] = true;
+          // go to next row w/ backtrack
+          dfs(r + 1);
+          // recover by undo
+          on_path[c] = diag1[r + c] = diag2[rc] = false;
+        }
+      }
+    };
+    // from first row: row 0
+    dfs(0);
+    return ans;
+  }
+
+  /**
+   * @brief LC1564: Greedy Algo: basic version
+   * but resize the warehouse vector
+   * Time: O(NlogN), Space: O(1)
+   *
+   * @param boxes
+   * @param warehouse
+   * @return int
+   */
+  int maxBoxesInWarehouse(vector<int>& boxes, vector<int>& warehouse) {
+    // preprocess warehouse height
+    for (int i = 1; i < warehouse.size(); i++) {
+      warehouse[i] = min(warehouse[i - 1], warehouse[i]);
+    }
+
+    sort(boxes.begin(), boxes.end());
+    int ans = 0;
+    for (int i = warehouse.size() - 1; i >= 0; i--) {
+      if (ans < boxes.size() && boxes[ans] <= warehouse[i]) {
+        ans++;
+      } }
+    return ans;
+  }
+  /**
+   * @brief Refined version of Greedy Algo
+   * Time: O(NlogN + m), Space: O(1)
+   *
+   * @param boxes
+   * @param warehouse
+   * @return int
+   */
+  int maxBoxesInWarehouse2(vector<int>& boxes, vector<int>& warehouse) {
+    int i = boxes.size() - 1;
+    int ans = 0;
+    sort(boxes.begin(), boxes.end());
+    for (int room : warehouse) {
+      // drop boxes not fit
+      while (i >= 0 && boxes[i] > room) {
+        i--;
+      }
+      if (i == -1) {
+        return ans;
+      }
+      ans++;
+      i--;
+    }
+    return ans;
+  }
+};