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文章目录
目录
文章目录
前言
一 排列、子集问题
1.1 全排列I
1.2 子集I
1.3 找出所有子集的异或总和
1.4 全排列II
1.5 字母大小写全排列
1.6 优美的排列
二 组合问题
2.1 电话号码的数字组合
2.2 括号生成
2.3 组合
2.4 目标和
2.5 组合总和
三 矩阵搜索问题
3.1 N皇后
3.2 有效的数独
3.3 解数独
3.4 单词搜素
3.5 黄金矿工
3.6 不同路径III
总结
前言
本篇详细介绍了进一步介绍DFS,让使用者对DFS有更加深刻的认知,而不是仅仅停留在表面,更好的模拟,为了更好的使用. 文章可能出现错误,如有请在评论区指正,让我们一起交流,共同进步!
我们在做DFS的题目时,首先要把决策树(通过策略把结果不重不漏的枚举得到)画下,缕清思路,设计代码自然水到渠成
一 排列、子集问题
1.1 全排列I
46. 全排列 - 力扣(LeetCode)
class Solution {
vector<vector<int>> ret;
vector<int> path;
bool check[7];
public:
vector<vector<int>> permute(vector<int>& nums) {
dfs(nums);
return ret;
}
void dfs(vector<int>& nums)
{
if(path.size() == nums.size())
{
ret.push_back(path);
return;
}
for(int i = 0;i<nums.size();i++)
{
if(check[i] == false)
{
path.push_back(nums[i]);
check[i] = true;
dfs(nums);
//回溯-> 恢复现场
path.pop_back();
check[i] = false;
}
}
}
};
1.2 子集I
78. 子集 - 力扣(LeetCode)
解法一:
class Solution {
vector<vector<int>> ret;
vector<int> path;
public:
vector<vector<int>> subsets(vector<int>& nums) {
dfs(nums,0);
return ret;
}
void dfs(vector<int>& nums,int n)
{
if(n == nums.size())
{
ret.push_back(path);
return;
}
//选
path.push_back(nums[n]);
dfs(nums,n+1);
path.pop_back();
//不选
dfs(nums,n+1);
}
};
解法二:
class Solution {
vector<vector<int>> ret;
vector<int> path;
public:
vector<vector<int>> subsets(vector<int>& nums) {
dfs(nums,0);
return ret;
}
void dfs(vector<int>& nums,int pos)
{
ret.push_back(path);
for(int i = pos;i<nums.size();i++)
{
path.push_back(nums[i]);
dfs(nums,i+1);
path.pop_back();
}
}
};
1.3 找出所有子集的异或总和
1863. 找出所有子集的异或总和再求和 - 力扣(LeetCode)
class Solution {
int sum;
int path;
public:
int subsetXORSum(vector<int>& nums) {
dfs(nums,0);
return sum;
}
void dfs(vector<int>& nums,int pos)
{
sum += path;
for(int i = pos;i<nums.size();i++)
{
path^=nums[i];
dfs(nums,i+1);
path^=nums[i];
}
}
};
1.4 全排列II
47. 全排列 II - 力扣(LeetCode)
class Solution {
vector<vector<int>> ret;
vector<int> path;
bool check[9];
public:
vector<vector<int>> permuteUnique(vector<int>& nums) {
sort(nums.begin(),nums.end());
dfs(nums);
return ret;
}
void dfs(vector<int>& nums)
{
if(path.size() == nums.size())
{
ret.push_back(path);
return;
}
for(int i = 0;i<nums.size();i++)
{
if(check[i] == false && (i==0 || nums[i] != nums[i-1] ||check[i-1] == true))
{
path.push_back(nums[i]);
check[i] = true;
dfs(nums);
path.pop_back();
check[i] = false;
}
}
}
};
1.5 字母大小写全排列
784. 字母大小写全排列 - 力扣(LeetCode)
class Solution {
vector<string> ret;
string path;
public:
vector<string> letterCasePermutation(string s) {
dfs(s,0);
return ret;
}
void dfs(string s,int pos)
{
if(pos == s.size())
{
ret.push_back(path);
return;
}
char ch = s[pos];
//不改变
path += ch;
dfs(s,pos+1);
path.pop_back();
//改变
if(ch<'0' || ch>'9')
{
char tmp = change(ch);
path += tmp;
dfs(s,pos+1);
path.pop_back();
}
}
char change(char ch)
{
if(ch>='a'&&ch<='z') ch-=32;
else ch+=32;
return ch;
}
};
1.6 优美的排列
526. 优美的排列 - 力扣(LeetCode)
class Solution {
bool check[16];
int ret;
public:
int countArrangement(int n) {
dfs(n,1);
return ret;
}
void dfs(int n, int pos)
{
if(pos == n+1)
{
ret++;
return;
}
for(int i = 1; i<=n;i++)
{
if(check[i] == false&&(i % pos == 0 || pos % i == 0))
{
check[i] = true;
dfs(n,pos+1);
check[i] = false;
}
}
}
};
二 组合问题
2.1 电话号码的数字组合
17. 电话号码的字母组合 - 力扣(LeetCode)
class Solution {
string path;
vector<string> ret;
vector<string> hash = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
public:
vector<string> letterCombinations(string digits) {
if(digits.size() == 0) return ret;
dfs(digits,0);
return ret;
}
void dfs(string digits,int pos)
{
if(pos == digits.size())
{
ret.push_back(path);
return;
}
for(auto& ch : hash[digits[pos] - '0'])
{
path.push_back(ch);
dfs(digits,pos+1);
path.pop_back();
}
}
};
2.2 括号生成
22. 括号生成 - 力扣(LeetCode)
class Solution {
vector<string> ret;
string path;
int count; //记录有效括号的对数
public:
vector<string> generateParenthesis(int n) {
count = n;
dfs(0,0);
return ret;
}
void dfs(int left,int right)
{
if(path.size() == 2*count)
{
ret.push_back(path);
return;
}
if(left<count)
{
path.push_back('(');
dfs(left+1,right);
path.pop_back();
}
if(right<left)
{
path.push_back(')');
dfs(left,right+1);
path.pop_back();
}
}
};
2.3 组合
77. 组合 - 力扣(LeetCode)
class Solution {
vector<vector<int>> ret;
vector<int> path;
int n, k;
public:
vector<vector<int>> combine(int _n, int _k) {
n = _n;
k = _k;
dfs(1);
return ret;
}
void dfs(int pos)
{
if(path.size() == k)
{
ret.push_back(path);
return;
}
for(int i = pos; i<=n; ++i)
{
path.push_back(i);
dfs(i+1);
path.pop_back();
}
}
};
2.4 目标和
494. 目标和 - 力扣(LeetCode)
class Solution {
int ret;
int target;
public:
int findTargetSumWays(vector<int>& nums, int _target) {
target = _target;
dfs(nums,0,0);
return ret;
}
void dfs(vector<int>& nums, int pos, int prev)
{
if(pos == nums.size())
{
if(prev == target)
ret++;
return;
}
dfs(nums,pos+1,prev+nums[pos]);
dfs(nums,pos+1,prev-nums[pos]);
}
};
2.5 组合总和
39. 组合总和 - 力扣(LeetCode)
解法一:
class Solution {
vector<vector<int>> ret;
vector<int> path;
int target;
public:
vector<vector<int>> combinationSum(vector<int>& candidates, int _target) {
target = _target;
dfs(candidates,0,0);
return ret;
}
void dfs(vector<int>& candidates,int sum, int pos)
{
if(sum>target) return;
if(sum == target)
{
ret.push_back(path);
return;
}
for(int i = pos;i<candidates.size();i++)
{
path.push_back(candidates[i]);
dfs(candidates,sum+candidates[i],i);
path.pop_back();
}
}
};
解法二:
class Solution {
vector<vector<int>> ret;
vector<int> path;
int target;
public:
vector<vector<int>> combinationSum(vector<int>& candidates, int _target) {
target = _target;
dfs(candidates,0,0);
return ret;
}
void dfs(vector<int>& candidates,int sum, int pos)
{
if(sum == target)
{
ret.push_back(path);
return;
}
if(sum>target || pos == candidates.size()) return;
for(int k = 0;k*candidates[pos]+sum<=target;k++)
{
if(k) path.push_back(candidates[pos]);
dfs(candidates,sum+k*candidates[pos],pos+1);
}
for(int k = 1;k*candidates[pos]+sum<=target;k++)
{
path.pop_back();
}
}
};
三 矩阵搜索问题
3.1 N皇后
51. N 皇后 - 力扣(LeetCode)
class Solution {
bool checkCol[10], checkDig1[20], checkDig2[20];
vector<vector<string>> ret;
vector<string> path;
public:
vector<vector<string>> solveNQueens(int n) {
path.resize(n);
for(int i = 0;i<n;i++)
path[i].append(n,'.');
dfs(n,0);
return ret;
}
void dfs(int n, int row)
{
if(row == n)
{
ret.push_back(path);
return;
}
for(int col = 0;col<n;col++)
{
if(!checkCol[col]&&!checkDig1[row-col+n]&&!checkDig2[row+col])
{
path[row][col] = 'Q';
checkCol[col] = checkDig1[row-col+n] = checkDig2[row+col] = true;
dfs(n,row+1);
path[row][col] = '.';
checkCol[col] = checkDig1[row-col+n] = checkDig2[row+col] = false;
}
}
}
};
3.2 有效的数独
36. 有效的数独 - 力扣(LeetCode)
class Solution {
bool row[9][10];
bool col[9][10];
bool grid[3][3][10];
public:
bool isValidSudoku(vector<vector<char>>& board) {
for(int i = 0;i<9;i++)
{
for(int j = 0;j<9;j++)
{
if(board[i][j] != '.')
{
int num = board[i][j] -'0';
if(row[i][num] || col[j][num] || grid[i/3][j/3][num])
return false;
row[i][num] = col[j][num] = grid[i/3][j/3][num] = true;
}
}
}
return true;
}
};
3.3 解数独
37. 解数独 - 力扣(LeetCode)
class Solution {
bool row[9][10];
bool col[9][10];
bool grid[3][3][10];
public:
void solveSudoku(vector<vector<char>>& board) {
for(int i = 0;i<9;i++)
{
for(int j = 0;j<9;j++)
{
if(board[i][j] != '.')
{
int num = board[i][j] -'0';
row[i][num] = col[j][num] = grid[i/3][j/3][num] = true;
}
}
}
dfs(board);
}
bool dfs(vector<vector<char>>& board)
{
for(int i = 0;i<9;i++)
{
for(int j = 0;j<9;j++)
{
if(board[i][j] == '.')
{
for(int num = 1; num<=9; num++)
{
if(!row[i][num]&&!col[j][num]&&!grid[i/3][j/3][num])
{
board[i][j] = num + '0';
row[i][num] = col[j][num] = grid[i/3][j/3][num] = true;
if(dfs(board) == true) return true; //判断当前所填的数是否有效
board[i][j] = '.';
row[i][num] = col[j][num] = grid[i/3][j/3][num] = false;
}
}
return false; //无法填数时,则说明之前的填的数错误,返回错误
}
}
}
return true;
}
};
3.4 单词搜素
79. 单词搜索 - 力扣(LeetCode)
class Solution {
bool vis[7][7];
int dx[4] = {0,0,1,-1};
int dy[4] = {1,-1,0,0};
int m,n;
public:
bool exist(vector<vector<char>>& board, string word) {
m = board.size(),n = board[0].size();
for(int i = 0;i<m;i++)
{
for(int j = 0;j<n;j++)
{
if(board[i][j] == word[0])
{
vis[i][j] = true;
if(dfs(board,word,i,j,1)) return true;
vis[i][j] = false;
}
}
}
return false;
}
bool dfs(vector<vector<char>>& board, string word,int i,int j,int pos)
{
if(pos == word.size()) return true;
for(int k = 0;k<4;k++)
{
int x = i + dx[k],y = j + dy[k];
if(x >=0 && x < m && y >= 0 && y < n && !vis[x][y] && board[x][y] == word[pos])
{
vis[x][y] = true;
if(dfs(board,word,x,y,pos+1)) return true;
vis[x][y] = false;
}
}
return false;
}
};
3.5 黄金矿工
1219. 黄金矿工 - 力扣(LeetCode)
本题的算法原理和单词搜索同,只不过多了一两个变量
class Solution {
bool vis[16][16];
int dx [4] = {0,0,1,-1};
int dy [4] = {1,-1,0,0};
int m,n,path,ret;
public:
int getMaximumGold(vector<vector<int>>& grid) {
m = grid.size(),n = grid[0].size();
for(int i = 0;i<m;i++)
{
for(int j = 0;j<n;j++)
{
if(grid[i][j] != 0)
{
vis[i][j] = true;
dfs(grid,i,j,grid[i][j]);
vis[i][j] = false;
}
}
}
return ret;
}
void dfs(vector<vector<int>>& grid,int i,int j,int path)
{
ret = max(ret,path);
for(int k = 0; k < 4; k++)
{
int x = i + dx[k], y = j + dy[k];
if(x >=0 && x < m && y >= 0 && y < n && !vis[x][y] && grid[x][y] != 0)
{
vis[x][y] = true;
dfs(grid,x,y,path+grid[x][y]);
vis[x][y] = false;
}
}
}
};
3.6 不同路径III
980. 不同路径 III - 力扣(LeetCode)
class Solution {
bool vis[21][21];
int dx[4] = {0,0,1,-1};
int dy[4] = {1,-1,0,0};
int m,n,step;
int ret;
public:
int uniquePathsIII(vector<vector<int>>& grid) {
int bx,by;
m = grid.size(),n = grid[0].size();
for(int i = 0;i<m;i++)
{
for(int j = 0; j < n;j++)
{
if(grid[i][j] == 0) step++;
else if(grid[i][j] == 1)
{
bx = i;
by = j;
}
}
}
step += 2;
vis[bx][by] = true;
dfs(grid,bx,by,1);
return ret;
}
void dfs(vector<vector<int>>& grid,int i,int j,int count)
{
if(grid[i][j] == 2)
{
if(count == step) //判断是否合法
ret++;
return;
}
for(int k = 0;k < 4;k++)
{
int x = i + dx[k],y = j + dy[k];
if(x >=0 && x < m && y >= 0 && y < n && !vis[x][y] && grid[x][y] != -1)
{
vis[x][y] = true;
dfs(grid,x,y,count + 1);
vis[x][y] = false;
}
}
}
};
总结
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