leetcode回溯法专栏（详解）
leetcode回溯法专栏
- 预备知识（递归）
- 回溯法
- LeetCode 17 电话号码的字母组合
- code1
- code2
- LeetCode 39 组合总和
- LeetCode 40 组合总和2
- LeetCode 46 全排列
- 图解
- code1
- code2
- leetcode 78. 子集
- code1
- code2
- LeetCode 79 单词搜索
- LeetCode 90 子集2
- LeetCode 93 复原IP地址
- LeetCode 131 分隔回文串
预备知识（递归）
#include
#include
using namespace std;
class Solution {
public:
void generation(int i, vector& nums, vector& item, vector>& result) {
if (i>=nums.size()) {
return;
}
item.push_back(nums[i]);
result.push_back(item);
generation(i+1, nums, item, result);
}
};
int main() {
vector nums;
nums.push_back(1);
nums.push_back(2);
nums.push_back(3);
vector item;
vector> result;
Solution().generation(0, nums, item, result);
for (int i = 0; i < result.size(); i++) {
for (int j = 0; j < result[i].size(); j++) {
cout << result[i][j] << " ";
}
cout << endl;
}
system("pause");
return 0;
}
回溯法
LeetCode 17 电话号码的字母组合
code1
#include
#include
#include
using namespace std;
class Solution {
public:
vector letterCombinations(string digits) {
if (digits.empty()) return {};
vector res;
vector dict{ " ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
letterCombinationsDFS(digits, dict, 0, "", res);
return res;
}
void letterCombinationsDFS(string& digits, vector& dict, int level, string out, vector& res) {
if (level == digits.size()) { res.push_back(out); return; }
string str = dict[digits[level] - '0'];//找到数字字符对应的字母
for (int i = 0; i < str.size(); ++i) {
letterCombinationsDFS(digits, dict, level + 1, out + str[i], res);
}
}
};
int main() {
vector res = Solution().letterCombinations("234");
for (int i = 0; i < res.size(); i++)
cout << res[i] << endl;
system("pause");
return 0;
}
code2
#include
#include
#include
#include
using namespace std;
class Solution {
private:
vector res;
const string letterMap[10] = {
" ",
"",
"abc",
"def",
"ghi",
"jkl",
"mno",
"pqrs",
"tuv",
"wxyz"
};
void findCombinations(const string& digits, int index, const string& s) {
if (index == digits.size()) {
res.push_back(s);
return;
}
char c = digits[index];
assert( c >= '0' && c <= '9' && c != '1');
string letters = letterMap[c - '0'];
for (int i = 0; i < letters.size(); ++i) {
findCombinations(digits,index+1,s+letters[i]);
}
}
public:
vector letterCombinations(string digits) {
res.clear();
if (digits == "") return res;
findCombinations(digits,0,"");
return res;
}
};
int main() {
string digits = "23";
vector res = Solution().letterCombinations(digits);
for (int i = 0; i < res.size(); ++i) {
cout << res[i] << endl;
}
system("pause");
return 0;
}
LeetCode 39 组合总和
#include
#include
#include
using namespace std;
class Solution {
public:
vector> combinationSum(vector& candidates, int target) {
vector> res;
vector out;
combinationSumDFS(candidates, target, 0, out, res);
return res;
}
void combinationSumDFS(vector& candidates, int target, int start, vector& out, vector>& res) {
if (target < 0) return;
if (target == 0) { res.push_back(out); return; }
for (int i = start; i < candidates.size(); ++i) {
out.push_back(candidates[i]);
combinationSumDFS(candidates, target - candidates[i], i, out, res);
out.pop_back();
}
}
};
int main() {
vector candidates = {2,3,6,7};
int target = 7;
vector> result = Solution().combinationSum(candidates, target);
for (int i = 0; i < result.size(); i++) {
for (int j = 0; j < result[i].size(); j++) {
cout << result[i][j] << " ";
}
cout << endl;
}
system("pause");
return 0;
}
LeetCode 40 组合总和2
#include
#include
#include
using namespace std;
class Solution {
public:
vector > combinationSum2(vector &num, int target) {
vector > res;
vector out;
sort(num.begin(), num.end());
combinationSum2DFS(num, target, 0, out, res);
return res;
}
void combinationSum2DFS(vector &num, int target, int start, vector &out, vector > &res) {
if (target < 0)
return;
else if (target == 0) res.push_back(out);
else {
for (int i = start; i < num.size(); ++i) {
if (i > start && num[i] == num[i - 1])
continue;
out.push_back(num[i]);
combinationSum2DFS(num, target - num[i], i, out, res);
out.pop_back();
}
}
}
};
int main() {
vector candidates = {2,2,2,2,1,7};
int target = 8;
vector> result = Solution().combinationSum2(candidates, target);
for (int i = 0; i < result.size(); i++) {
for (int j = 0; j < result[i].size(); j++) {
cout << result[i][j] << " ";
}
cout << endl;
}
system("pause");
return 0;
}
LeetCode 46 全排列
图解
code1
#include
#include
using namespace std;
class Solution {
public:
vector> permute(vector& num) {
vector> res;
permuteDFS(num, 0, res);
return res;
}
void permuteDFS(vector& num, int start, vector>& res) {
if (start >= num.size()) res.push_back(num);
for (int i = start; i < num.size(); ++i) {
swap(num[start], num[i]);
permuteDFS(num, start + 1, res);
swap(num[start], num[i]);
}
}
};
int main() {
vector num{1,2,3};
vector> res = Solution().permute(num);
for (int i = 0; i < res.size(); i++) {
for (int j = 0; j < res[i].size(); j++)
cout << res[i][j];
cout << endl;
}
system("pause");
return 0;
}
code2
#include
#include
#include
using namespace std;
class Solution {
private:
vector> res;
vector used;
void generatePermutation(const vector& nums, int index, vector& p) {
if (index == nums.size()) {
res.push_back(p);
return;
}
for (int i = 0; i < nums.size(); i++) {
if (!used[i]) {
p.push_back(nums[i]);
used[i] = true;
generatePermutation(nums, index + 1, p);
p.pop_back();
used[i] = false;
}
}
return;
}
public:
vector> permute(vector& nums) {
res.clear();
if (nums.size() == 0)
return res;
used = vector(nums.size(), false);
vector p;
generatePermutation(nums, 0, p);
return res;
}
};
int main() {
vector num{1, 2, 3};
vector> res = Solution().permute(num);
for (int i = 0; i < res.size(); i++) {
for (int j = 0; j < res[i].size(); j++)
cout << res[i][j];
cout << endl;
}
system("pause");
return 0;
}
leetcode 78. 子集
code1
#include
#include
using namespace std;
class Solution {
private:
void generation(int i, vector& nums, vector& item, vector>& result) {
if (i>=nums.size()) {
return;
}
item.push_back(nums[i]);
result.push_back(item);
generation(i + 1, nums, item, result);
item.pop_back();
generation(i + 1, nums, item, result);
}
public:
vector> subsets(vector& nums) {
vector item;
vector> result;
result.push_back(item);
generation(0, nums, item, result);
return result;
}
};
int main() {
vector nums;
nums.push_back(1);
nums.push_back(2);
nums.push_back(3);
vector> result = Solution().subsets(nums);
for (int i = 0; i < result.size(); i++) {
for (int j = 0; j < result[i].size(); j++) {
cout << result[i][j] << " ";
}
cout << endl;
}
system("pause");
return 0;
}
code2
#include
#include
using namespace std;
class Solution {
private:
void generation(int pos, vector& nums, vector& item, vector>& result) {
result.push_back(item);
for (int i = pos; i < nums.size(); i++) {
item.push_back(nums[i]);
generation(i + 1, nums, item, result);
item.pop_back();
}
}
public:
vector> subsets(vector& nums) {
vector item;
vector> result;
generation(0, nums, item, result);
return result;
}
};
int main() {
vector nums;
nums.push_back(1);
nums.push_back(2);
nums.push_back(3);
vector> result = Solution().subsets(nums);
for (int i = 0; i < result.size(); i++) {
for (int j = 0; j < result[i].size(); j++) {
cout < LeetCode 79 单词搜索
#include
#include
#include
using namespace std;
class Solution {
public:
bool exist(vector>& board, string word) {
if (board.empty() || board[0].empty()) return false;
int m = board.size(), n = board[0].size();
vector> visited(m, vector(n));
for (int i = 0; i < m; ++i) {
for (int j = 0; j < n; ++j) {
if (search(board, word, 0, i, j, visited)) return true;
}
}
return false;
}
bool search(vector>& board, string word, int idx, int i, int j, vector>& visited) {
if (idx == word.size()) return true;
int m = board.size(), n = board[0].size();
if (i < 0 || j < 0 || i >= m || j >= n || visited[i][j] || board[i][j] != word[idx]) return false;
visited[i][j] = true;
bool res = search(board, word, idx + 1, i - 1, j, visited)
|| search(board, word, idx + 1, i + 1, j, visited)
|| search(board, word, idx + 1, i, j - 1, visited)
|| search(board, word, idx + 1, i, j + 1, visited);
visited[i][j] = false;
return res;
}
};
int main() {
vector> board =
{
{'A', 'B', 'C', 'E'},
{'S', 'F', 'C', 'S'},
{'A', 'D', 'E', 'E'}
};
string word = "ABCCED";
bool res = Solution().exist(board, word);
cout << res << endl;
system("pause");
return 0;
}
LeetCode 90 子集2
#include
#include
#include
using namespace std;
class Solution {
public:
vector> subsetsWithDup(vector &S) {
if (S.empty()) return {};
vector> res;
vector out;
sort(S.begin(), S.end());
getSubsets(S, 0, out, res);
return res;
}
void getSubsets(vector &S, int pos, vector &out, vector> &res) {
res.push_back(out);
cout << endl;
for (int i = pos; i < S.size(); ++i) {
out.push_back(S[i]);
getSubsets(S, i + 1, out, res);
out.pop_back();
while (i + 1 < S.size() && S[i] == S[i + 1])
++i;
}
}
};
int main() {
vector nums = {2,1,2,2};
vector> result = Solution().subsetsWithDup(nums);
for (int i = 0; i < result.size(); i++) {
for (int j = 0; j < result[i].size(); j++) {
cout << result[i][j] << " ";
}
cout << endl;
}
system("pause");
return 0;
}
LeetCode 93 复原IP地址
#include
#include
#include
#include
using namespace std;
class Solution {
public:
vector restoreIpAddresses(string s) {
vector res;
restore(s, 4, "", res);
return res;
}
void restore(string s, int k, string out, vector &res) {
if (k == 0) {
if (s.empty()) res.push_back(out);
}
else {
for (int i = 1; i <= 3; ++i) {
if (s.size() >= i && isValid(s.substr(0, i))) {
if (k == 1) restore(s.substr(i), k - 1, out + s.substr(0, i), res);
else restore(s.substr(i), k - 1, out + s.substr(0, i) + ".", res);
}
}
}
}
bool isValid(string s) {
if (s.empty() || s.size() > 3 || (s.size() > 1 && s[0] == '0')) return false;
int res = atoi(s.c_str());
return res <= 255 && res >= 0;
}
};
int main() {
string s = "25525511135";
vector result = Solution().restoreIpAddresses(s);
for (int i = 0; i < result.size(); i++) {
cout << result[i] << endl;
}
system("pause");
return 0;
}
LeetCode 131 分隔回文串
如果原字符串是 abcd, 那么访问顺序为: a -> b -> c -> d -> cd -> bc -> bcd-> ab -> abc -> abcd, 这是对于没有两个或两个以上子回文串的情况。那么假如原字符串是 aabc，那么访问顺序为：a -> a -> b -> c -> bc -> ab -> abc -> aa -> b -> c -> bc -> aab -> aabc。
#include
#include
#include
using namespace std;
class Solution {
public:
vector> partition(string s) {
vector> res;
vector out;
helper(s, 0, out, res);
return res;
}
void helper(string s, int start, vector& out, vector>& res) {
if (start == s.size()) {
res.push_back(out);
return;
}
for (int i = start; i < s.size(); ++i) {
if (!isPalindrome(s, start, i)) continue;
out.push_back(s.substr(start, i - start + 1));
helper(s, i + 1, out, res);
out.pop_back();
}
}
bool isPalindrome(string s, int start, int end) {
while (start < end) {
if (s[start] != s[end]) return false;
++start; --end;
}
return true;
}
};
int main() {
string s = "aabc";
vector> res = Solution().partition(s);
for (int i = 0; i < res.size(); i++) {
for (int j = 0; j < res[i].size(); j++) {
cout << res[i][j] << " ";
}
}
system("pause");
return 0;
}