代码随想录二刷day30

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一、力扣332. 重新安排行程

class Solution {
    private LinkedList<String> res;
    private LinkedList<String> path = new LinkedList<>();

    public List<String> findItinerary(List<List<String>> tickets) {
        Collections.sort(tickets, (a, b) -> a.get(1).compareTo(b.get(1)));
        path.add("JFK");
        boolean[] used = new boolean[tickets.size()];
        backTracking((ArrayList) tickets, used);
        return res;
    }

    public boolean backTracking(ArrayList<List<String>> tickets, boolean[] used) {
        if (path.size() == tickets.size() + 1) {
            res = new LinkedList(path);
            return true;
        }

        for (int i = 0; i < tickets.size(); i++) {
            if (!used[i] && tickets.get(i).get(0).equals(path.getLast())) {
                path.add(tickets.get(i).get(1));
                used[i] = true;

                if (backTracking(tickets, used)) {
                    return true;
                }

                used[i] = false;
                path.removeLast();
            }
        }
        return false;
    }
}

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二、力扣51. N 皇后

class Solution {
    List<List<String>> res = new ArrayList<>();
    public List<List<String>> solveNQueens(int n) {
        char[][] ch = new char[n][n];
        for(char[] c:ch){
            Arrays.fill(c, '.');
        }
        fun(0, n, ch);
        return res;
    }
    public void fun(int row, int n, char[][] ch){
        if(row == n){
            res.add(new ArrayList<>(temp(ch)));
            return;
        }
        for(int col = 0; col < n; col ++){
            if(flag(ch, row, col, n)){
                ch[row][col] = 'Q';
                fun(row +1, n, ch);
                ch[row][col] = '.';
            }
        }
    }
    public List<String> temp(char[][] ch){
        List<String> li = new ArrayList<>();
        for(char[] c: ch){
            li.add(String.copyValueOf(c));
        }
        return li;
    }
    public boolean flag(char[][] ch, int row, int col, int n){
        //判断行
        for(int i = 0; i < col; i ++){
            if(ch[row][i] == 'Q'){
                return false;
            }
        }
        //判断列
        for(int i = 0; i < row; i ++){
            if(ch[i][col] == 'Q'){
                return false;
            }
        }
        //判断°角
        for(int i = row-1, j = col-1; i >= 0 && j >= 0; i --, j --){
            if(ch[i][j] == 'Q'){
                return false;
            }
        }
        //判断135度角
        for(int i = row-1, j = col+1; i >= 0 && j <= n-1; i--, j ++){
            if(ch[i][j] == 'Q'){
                return false;
            }
        }
        return true;
    }
}
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三、力扣37. 解数独

class Solution {
    public void solveSudoku(char[][] board) {
        fun(board);
    }
    public boolean fun(char[][] board){
        for(int i = 0; i < 9; i ++){
            for(int j = 0; j < 9; j ++){
                if(board[i][j] != '.'){
                    continue;
                }
                for(char k = '1'; k <= '9'; k ++){
                    if(flag(board, i, j, k)){
                        board[i][j] = k;
                        if(fun(board)){
                            return true;
                        }
                        board[i][j] = '.';
                    }
                }
                return false;
            }
        }
        return true;
    }
    public boolean flag(char[][] board, int row, int col, char k){
        //判断行
        for(int i = 0; i < 9; i ++){
            if(board[row][i] == k){
                return false;
            }
        }
        //判断列
        for(int i = 0; i < 9; i ++){
            if(board[i][col] == k){
                return false;
            }
        }
        //判断9宫格
        int a = (row/3)*3;
        int b = (col/3)*3;
        for(int i = a; i < a + 3; i ++){
            for(int j = b; j < b + 3; j ++){
                if(board[i][j] == k){
                    return false;
                }
            }
        }
        return true;
    }
}```

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