332.重新安排行程

题目链接：332.重新安排行程
文档链接：332.重新安排行程

C++实现

class Solution {
private: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;target.second++;result.pop_back();}}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]]++;}result.push_back("JFK");backtracking(tickets.size(), result);return result;}
};

51. N皇后

题目链接：51. N皇后
文档链接：51. N皇后
视频链接：这就是传说中的N皇后？ 回溯算法安排！| LeetCode：51.N皇后

C++实现

class Solution {
private:vector<vector<string>> result;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;}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] = '.';}}}public:vector<vector<string>> solveNQueens(int n) {result.clear();vector<string> chessboard(n, string(n,'.'));backtracking(n, 0, chessboard);return result;}
};

37. 解数独

题目链接：37. 解数独
文档链接：37. 解数独
视频链接：回溯算法二维递归？解数独不过如此！| LeetCode：37. 解数独

C++实现

class Solution {
private: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 i = 0; i<9;i++){if(board[i][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;}bool backtarcking(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(int k = '1'; k<='9';k++){if(isValid(i,j,k,board)){board[i][j] = k;if(backtarcking(board)) return true;board[i][j] = '.';}}return false;}}}return true;}public:void solveSudoku(vector<vector<char>>& board) {backtarcking(board);}
};