代码随想录训练营二刷第三十天 | 332.重新安排行程 51. N皇后 37. 解数独

代码随想录训练营二刷第三十天 | 332.重新安排行程 51. N皇后 37. 解数独

一、332.重新安排行程

题目链接：https://leetcode.cn/problems/reconstruct-itinerary/
思路：直接看题解了，没太想明白为什么需要排序

class Solution {
    private Deque<String> res;
    private Map<String, Map<String, Integer>> map;

    private boolean backTracking(int ticketNum){
        if(res.size() == ticketNum + 1){
            return true;
        }
        String last = res.getLast();
        if(map.containsKey(last)){//防止出现null
            for(Map.Entry<String, Integer> target : map.get(last).entrySet()){
                int count = target.getValue();
                if(count > 0){
                    res.add(target.getKey());
                    target.setValue(count - 1);
                    if(backTracking(ticketNum)) return true;
                    res.removeLast();
                    target.setValue(count);
                }
            }
        }
        return false;
    }

    public List<String> findItinerary(List<List<String>> tickets) {
        map = new HashMap<String, Map<String, Integer>>();
        res = new LinkedList<>();
        for(List<String> t : tickets){
            Map<String, Integer> temp;
            if(map.containsKey(t.get(0))){
                temp = map.get(t.get(0));
                temp.put(t.get(1), temp.getOrDefault(t.get(1), 0) + 1);
            }else{
                temp = new TreeMap<>();//升序Map
                temp.put(t.get(1), 1);
            }
            map.put(t.get(0), temp);

        }
        res.add("JFK");
        backTracking(tickets.size());
        return new ArrayList<>(res);
    }
}

二、51. N皇后

题目链接：https://leetcode.cn/problems/n-queens/
思路：题目不难主要要知道N皇后的限制，不能同行同列，斜着45度135度都不行。这些是判断条件。然后for循环控制每层是n，递归深度也是n，每层递归处理一行。

class Solution {
   List<List<String>> arrayLists = new ArrayList<>();
    public List<List<String>> solveNQueens(int n) {
        char[][] queens = new char[n][n];
        for (int i = 0; i < n; i++) {
            Arrays.fill(queens[i], '.');
        }
        backTracking(0, n, queens);
        return arrayLists;
    }
    void backTracking(int row, int n, char[][] queens) {
        if (row == n) {
            arrayLists.add(arrayToList(queens));
            return;
        }
        for (int i = 0; i < n; i++) {
            if (isTrue(queens, row, i, n)) {
                queens[row][i] = 'Q';
                backTracking(row+1, n, queens);
                queens[row][i] = '.';
            }
        }
    }
    List<String> arrayToList(char[][] queens) {
        List<String> list = new ArrayList<>();
        for (char[] queen : queens) {
            list.add(String.copyValueOf(queen));
        }
        return list;
    }
    boolean isTrue(char[][] queens, int row, int col, int n) {
        for (int i = 0; i < n; i++) {
            if (queens[i][col] == 'Q') {
                return false;
            }
        }
        for (int i = row-1, j = col-1; i >= 0 && j >= 0; i--, j--) {
            if (queens[i][j] == 'Q') {
                return false;
            }
        }
        for (int i = row-1, j = col+1; i >= 0 && j < n; i--, j++) {
            if (queens[i][j] == 'Q') {
                return false;
            }
        }
        return true;
    }
}

三、37. 解数独

题目链接：https://leetcode.cn/problems/sudoku-solver/
思路：二维递归

class Solution {
    public void solveSudoku(char[][] board) {
        backTracking(board);
    }
    boolean backTracking(char[][] board) {
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board.length; j++) {
                if (board[i][j] != '.')continue;
                for (char k = '1'; k <= '9'; k++) {
                    if (isTrue(board, i, j, k)) {
                        board[i][j] = k;
                        if (backTracking(board))return true;
                        board[i][j] = '.';
                    }
                }
                return false;
            }
        }
        return true;
    }

    boolean isTrue(char[][] board, int row, int col, int val) {
        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 a = (row/3) * 3, b = (col/3)*3;
        for (int i = a; i < a + 3; i++) {
            for (int j = b; j < b + 3; j++) {
                if (board[i][j] == val) return false;
            }
        }
        return true;
    }
}