代码随想录算法训练营三刷day30|回溯之 332.重新安排行程 51. N皇后 37. 解数独

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三刷day30

- - 332.重新安排行程
  - 51. N皇后
  - 37. 解数独

332.重新安排行程

题目链接

class Solution {
private:
// unordered_map<出发机场, map<到达机场, 航班次数>> targets
unordered_map<string, map<string, int>> targets;
bool backtracking(int ticketNum, vector<string>& result) {if (result.size() == ticketNum + 1) {return true;}for (pair<const string, int>& target : targets[result[result.size() - 1]]) {if (target.second > 0 ) { // 记录到达机场是否飞过了result.push_back(target.first);target.second--;if (backtracking(ticketNum, result)) return true;result.pop_back();target.second++;}}return false;
}
public:vector<string> findItinerary(vector<vector<string>>& tickets) {targets.clear();vector<string> result;for (const vector<string>& vec : tickets) {targets[vec[0]][vec[1]]++; // 记录映射关系}result.push_back("JFK"); // 起始机场backtracking(tickets.size(), result);return result;}
};

51. N皇后

题目链接

class Solution {
private:
vector<vector<string>> result;
// n 为输入的棋盘大小
// row 是当前递归到棋盘的第几行了
void backtracking(int n, int row, vector<string>& chessboard) {if (row == n) {result.push_back(chessboard);return;}for (int col = 0; col < n; col++) {if (isValid(row, col, chessboard, n)) { // 验证合法就可以放chessboard[row][col] = 'Q'; // 放置皇后backtracking(n, row + 1, chessboard);chessboard[row][col] = '.'; // 回溯，撤销皇后}}
}
bool isValid(int row, int col, vector<string>& chessboard, int n) {// 检查列for (int i = 0; i < row; i++) { // 这是一个剪枝if (chessboard[i][col] == 'Q') {return false;}}// 检查 45度角是否有皇后for (int i = row - 1, j = col - 1; i >=0 && j >= 0; i--, j--) {if (chessboard[i][j] == 'Q') {return false;}}// 检查 135度角是否有皇后for(int i = row - 1, j = col + 1; i >= 0 && j < n; i--, j++) {if (chessboard[i][j] == 'Q') {return false;}}return true;
}
public:vector<vector<string>> solveNQueens(int n) {result.clear();std::vector<std::string> chessboard(n, std::string(n, '.'));backtracking(n, 0, chessboard);return result;}

37. 解数独

题目链接

class Solution {
private:
bool backtracking(vector<vector<char>>& board) {for (int i = 0; i < board.size(); i++) {        // 遍历行for (int j = 0; j < board[0].size(); j++) { // 遍历列if (board[i][j] == '.') {for (char k = '1'; k <= '9'; k++) {     // (i, j) 这个位置放k是否合适if (isValid(i, j, k, board)) {board[i][j] = k;                // 放置kif (backtracking(board)) return true; // 如果找到合适一组立刻返回board[i][j] = '.';              // 回溯，撤销k}}return false;  // 9个数都试完了，都不行，那么就返回false }                }}return true; // 遍历完没有返回false，说明找到了合适棋盘位置了
}
bool isValid(int row, int col, char val, vector<vector<char>>& board) {for (int i = 0; i < 9; i++) { // 判断行里是否重复if (board[row][i] == val) {return false;}}for (int j = 0; j < 9; j++) { // 判断列里是否重复if (board[j][col] == val) {return false;}}int startRow = (row / 3) * 3;int startCol = (col / 3) * 3;for (int i = startRow; i < startRow + 3; i++) { // 判断9方格里是否重复for (int j = startCol; j < startCol + 3; j++) {if (board[i][j] == val ) {return false;}}}return true;
}
public:void solveSudoku(vector<vector<char>>& board) {backtracking(board);}
};