算法训练营30天|LeetCode 332.重新安排行程 51.N皇后 37.解数独

LeetCode 332.重新安排行程

题目链接：

LeetCode 332.重新安排行程

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 ) { // 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;
    }

    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;
    }
};

LeetCode 51.N皇后

题目链接：

LeetCode 51.N皇后

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) {
        vector<string>chessboard(n,string(n,'.'));
        backtracking(n,0,chessboard);
        return result;
    }
};

LeetCode 37.解数独

题目链接：

LeetCode 37.解数独

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++) { // (i, j) 这个位置放k是否合适
                        if (isValid(i, j, k, board)) {
                            board[i][j] = k; // 放置k
                            if (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;
    }
    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;
    }
    void solveSudoku(vector<vector<char>>& board) { backtracking(board); }
};