Day 30 | 332. Reconstruct Itinerary | 51. N-Queens | 37. Sudoku Solver

Day 1 | 704. Binary Search | 27. Remove Element | 35. Search Insert Position | 34. First and Last Position of Element in Sorted Array
Day 2 | 977. Squares of a Sorted Array | 209. Minimum Size Subarray Sum | 59. Spiral Matrix II
Day 3 | 203. Remove Linked List Elements | 707. Design Linked List | 206. Reverse Linked List
Day 4 | 24. Swap Nodes in Pairs| 19. Remove Nth Node From End of List| 160.Intersection of Two Lists
Day 6 | 242. Valid Anagram | 349. Intersection of Two Arrays | 202. Happy Numbe | 1. Two Sum
Day 7 | 454. 4Sum II | 383. Ransom Note | 15. 3Sum | 18. 4Sum
Day 8 | 344. Reverse String | 541. Reverse String II | 替换空格 | 151.Reverse Words in a String | 左旋转字符串
Day 9 | 28. Find the Index of the First Occurrence in a String | 459. Repeated Substring Pattern
Day 10 | 232. Implement Queue using Stacks | 225. Implement Stack using Queue
Day 11 | 20. Valid Parentheses | 1047. Remove All Adjacent Duplicates In String | 150. Evaluate RPN
Day 13 | 239. Sliding Window Maximum | 347. Top K Frequent Elements
Day 14 | 144.Binary Tree Preorder Traversal | 94.Binary Tree Inorder Traversal| 145.Binary Tree Postorder Traversal
Day 15 | 102. Binary Tree Level Order Traversal | 226. Invert Binary Tree | 101. Symmetric Tree
Day 16 | 104.MaximumDepth of BinaryTree| 111.MinimumDepth of BinaryTree| 222.CountComplete TreeNodes
Day 17 | 110. Balanced Binary Tree | 257. Binary Tree Paths | 404. Sum of Left Leaves
Day 18 | 513. Find Bottom Left Tree Value | 112. Path Sum | 105&106. Construct Binary Tree
Day 20 | 654. Maximum Binary Tree | 617. Merge Two Binary Trees | 700.Search in a Binary Search Tree
Day 21 | 530. Minimum Absolute Difference in BST | 501. Find Mode in Binary Search Tree | 236. Lowes
Day 22 | 235. Lowest Common Ancestor of a BST | 701. Insert into a BST | 450. Delete Node in a BST
Day 23 | 669. Trim a BST | 108. Convert Sorted Array to BST | 538. Convert BST to Greater Tree
Day 24 | 77. Combinations
Day 25 | 216. Combination Sum III | 17. Letter Combinations of a Phone Number
Day 27 | 39. Combination Sum | 40. Combination Sum II | 131. Palindrome Partitioning
Day 28 | 93. Restore IP Addresses | 78. Subsets | 90. Subsets II
Day 29 | 491. Non-decreasing Subsequences | 46. Permutations | 47. Permutations II

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LeetCode 332. Reconstruct Itinerary

Question Link

class Solution {
    List<String> result = new ArrayList<>();
    LinkedList<String> path = new LinkedList<>();
    boolean[] used;

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

    boolean backTracking(List<List<String>> tickets){
        if(path.size() == tickets.size() + 1){
            result = new ArrayList<>(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))
                    return true;
                used[i] = false;
                path.removeLast();
            }
        }
        return false;
    }
}

• Terminal condition: when the number of airports equals the number of routes + 1

LeetCode 51. N-Queens

Question Link

class Solution {
    List<List<String>> res = new ArrayList<>();

    public List<List<String>> solveNQueens(int n) {
        char[][] chessboard = new char[n][n];
        for(char[] c : chessboard)
            Arrays.fill(c, '.');
        backTracing(n, 0, chessboard);
        return res;
    }

    void backTracing(int n, int row, char[][] chessboard){
        if(row == n){
            res.add(array2List(chessboard));
            return;
        }
        for(int col = 0; col < n; col++){
            if(isValid(col, row, n, chessboard)){
                chessboard[row][col] = 'Q';
                backTracing(n, row + 1, chessboard);
                chessboard[row][col] = '.';
            }
        }
    }

    boolean isValid(int col, int row, int n, char[][] chessboard){
        // check the column
        for(int i = 0; i < row; i++){
            if(chessboard[i][col] == 'Q')
                return false;
        }
        // check the 45° diagonal
        for(int i = row - 1, j = col - 1; i >= 0 && j >= 0; i--, j--){
            if(chessboard[i][j] == 'Q')
                return false;
        }
        // check the 135° diagonal
        for(int i = row - 1, j = col + 1; i >= 0 && j < n; i--, j++){
            if(chessboard[i][j] == 'Q')
                return false;
        }
        return true;
    }

    List<String> array2List(char[][] chessboard){
        List<String> list = new ArrayList<>();
        for(char[] c : chessboard)
            list.add(String.copyValueOf(c));
        return list;
    }

}

• Go from top to bottom, left to right to verfy each position.

LeetCode 37. Sudoku Solver

Question Link

class Solution {
    public void solveSudoku(char[][] board) {
        backTracing(board);
    }

    boolean backTracing(char[][] board){
        for(int i = 0; i < 9; i++){          // Traverse over rows
            for(int j = 0; j < 9; j++){      // Traverse over columns
                if(board[i][j] != '.')       // Skip the non-empty cells
                    continue;
                for(char k = '1'; k <= '9'; k++){   
                    if(isValidSudoku(i, j, k, board)){
                        board[i][j] = k;
                        if(backTracing(board))
                            return true;
                        board[i][j] = '.';
                    }
                }
                // If 1-9 is invalid 
                return false;
            }
        }
        return true;
    }

    boolean isValidSudoku(int row, int col, int k, char[][] board){
        // Check whether the data of the same row is duplicated
        for(int i = 0; i < 9; i++){
            if(board[row][i] == k)
                return false;
        }
        // Check whether the data of the same column is duplicated
        for(int j = 0; j < 9; j++){
            if(board[j][col] == k)
                return false;
        }
        // Check whether the data of the 9x9 matric is duplicated
        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] == k)
                    return false;
            }
        }
        return true;
    }
}

• In backtracing, the return value is void if we collect all paths. If we only collect one path, the return value should be boolean.