Problem:

Write a program to solve a Sudoku puzzle by filling the empty cells.

Empty cells are indicated by the character '.'.

You may assume that there will be only one unique solution.

A sudoku puzzle…

…and its solution numbers marked in red.

Idea:

DFS + backtracking

Solution:

C++

// Author: Huahua
// Runtime: 3 ms
class Solution {
public:
    void solveSudoku(vector<vector<char>>& board) {
        rows_ = vector<vector<int>>(9, vector<int>(10));
        cols_ = vector<vector<int>>(9, vector<int>(10));
        boxes_ = vector<vector<int>>(9, vector<int>(10));
        
        for(int i = 0; i < 9; i++)
            for(int j = 0; j < 9; j++) {
                const char c = board[i][j];                
                if ( c != '.') {
                    int n = c - '0';                    
                    int bx = j / 3;
                    int by = i / 3;
                    rows_[i][n] = 1;
                    cols_[j][n] = 1;
                    boxes_[by * 3 + bx][n] = 1;
                }
            }
        
        fill(board, 0, 0);
    }
    
private:
    vector<vector<int>> rows_, cols_, boxes_;
    
    bool fill(vector<vector<char>>& board, int x, int y) {
        if (y == 9)
            return true;
        
        int nx = (x + 1) % 9;
        int ny = (nx == 0) ? y + 1 : y;
        
        if (board[y][x] != '.') return fill(board, nx, ny);
        
        for (int i = 1; i <= 9; i++) {
            int bx = x / 3;
            int by = y / 3;
            int box_key = by * 3 + bx;
            if (!rows_[y][i] && !cols_[x][i] && !boxes_[box_key][i]) {
                rows_[y][i] = 1;
                cols_[x][i] = 1;
                boxes_[box_key][i] = 1;
                board[y][x] = i + '0';
                if (fill(board, nx, ny)) return true;
                board[y][x] = '.';
                boxes_[box_key][i] = 0;
                cols_[x][i] = 0;
                rows_[y][i] = 0;
            }
        }
        return false;
    }
};

Related Problems:

• [解题报告] LeetCode 51. N-Queens

The post 花花酱 LeetCode 37. Sudoku Solver appeared first on Huahua's Tech Road.

Problem:

[
  [1, 7],
  [1, 2, 5],
  [2, 6],
  [1, 1, 6]
]

Idea:

DFS

Solution:

C++ / set

// Author: Huahua
// Runtime: 9 ms
class Solution {
public:
    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
        set<vector<int>> ans;
        std::sort(candidates.begin(), candidates.end());
        vector<int> curr;
        dfs(candidates, target, 0, ans, curr);
        return vector<vector<int>>(ans.begin(), ans.end());
    }
private:
    void dfs(const vector<int>& candidates, 
             int target, int s, 
             set<vector<int>>& ans,              
             vector<int>& curr) {
        if (target == 0) {
            ans.insert(curr);
            return;
        }
        
        for (int i = s; i < candidates.size(); ++i) {
            int num = candidates[i];
            if (num > target) return;
            curr.push_back(num);
            dfs(candidates, target - num, i + 1, ans, curr);
            curr.pop_back();
        }
    }
};

C++ / vector

// Author: Huahua
// Runtime: 9ms

class Solution {
public:
    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
        vector<vector<int>> ans;
        std::sort(candidates.begin(), candidates.end());
        vector<int> curr;
        dfs(candidates, target, 0, ans, curr);
        return ans;
    }
private:
    void dfs(const vector<int>& candidates, 
             int target, int s, 
             vector<vector<int>>& ans,              
             vector<int>& curr) {
        if (target == 0) {
            ans.push_back(curr);
            return;
        }
        
        for (int i = s; i < candidates.size(); ++i) {
            int num = candidates[i];
            if (num > target) return;
            if (i > s && candidates[i] == candidates[i - 1]) continue;
            curr.push_back(num);
            dfs(candidates, target - num, i + 1, ans, curr);
            curr.pop_back();
        }
    }
};

The post 花花酱 LeetCode 40. Combination Sum II appeared first on Huahua's Tech Road.