代码随想录算法训练营第三十天|332.重新安排行程、51. N皇后、37. 解数独

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332.重新安排行程

题目链接：力扣（LeetCode）官网 - 全球极客挚爱的技术成长平台
文档讲解：代码随想录

C++代码：

class Solution {
public:
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 ) {
                result.push_back(target.first);
                target.second--;
                if (backtracking(ticketNum, result)) return true;
                result.pop_back();
                target.second++;
            }
        }
        return false;
    }
    vector<string> findItinerary(vector<vector<string>>& tickets) {
        targets.clear();
        vector<string> result;
        for (const vector<string>& vec : tickets) {
            targets[vec[0]][vec[1]]++;
        }
        result.push_back("JFK");
        backtracking(tickets.size(), result);
        return result;
    }
};

51. N皇后

题目链接：力扣（LeetCode）官网 - 全球极客挚爱的技术成长平台
文档讲解：代码随想录
视频讲解：这就是传说中的N皇后？回溯算法安排！| LeetCode：51.N皇后_哔哩哔哩_bilibili

C++代码：

class Solution {
public:
    vector<vector<string>> result;
    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)) {
                chessboard[row][col] = 'Q';
                backtracking(n, row + 1, chessboard);
                chessboard[row][col] = '.';
            }
        }
    }
    bool isValid(int row, int col, vector<string>& chessboard, int n) {
        for (int i = 0; i < row; i++) {
            if (chessboard[i][col] == 'Q') {
                return false;
            }
        }
        for (int i = row - 1, j = col - 1; i >=0 && j >= 0; i--, j--) {
            if (chessboard[i][j] == 'Q') {
                return false;
            }
        }
        for(int i = row - 1, j = col + 1; i >= 0 && j < n; i--, j++) {
            if (chessboard[i][j] == 'Q') {
                return false;
            }
        }
        return true;
    }
    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：力扣（LeetCode）官网 - 全球极客挚爱的技术成长平台
文档讲解：代码随想录
视频讲解：回溯算法二维递归？解数独不过如此！| LeetCode：37. 解数独_哔哩哔哩_bilibili

C++代码：

class Solution {
public:
    bool backtracking(vector<vector<char>>& board) {
        for (int i = 0; i < board.size(); i++) {
            for (int j = 0; j < board[0].size(); j++) {
                if (board[i][j] == '.') {
                    for (char k = '1'; k <= '9'; k++) {
                        if (isValid(i, j, k, board)) {
                            board[i][j] = k;
                            if (backtracking(board)) return true;
                            board[i][j] = '.';
                        }
                    }
                    return false;
                }
            }
        }
        return true;
    }
    bool isValid(int row, int col, char val, vector<vector<char>>& board) {
        for (int i = 0; i < 9; i++) {
            if (board[row][i] == val) {
                return false;
            }
        }
        for (int j = 0; j < 9; j++) {
            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++) {
            for (int j = startCol; j < startCol + 3; j++) {
                if (board[i][j] == val ) {
                    return false;
                }
            }
        }
        return true;
    }
    void solveSudoku(vector<vector<char>>& board) {
        backtracking(board);
    }
};