2022暑假算法训练第18日

[a_a]

title: 算法训练第18日
date: 2022-07-19 21:35:24
summary:
tags:
categories:

组合总和 II

class Solution {
private:
    vector<vector<int>> res;
public:
    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
        sort(candidates.begin() ,candidates.end());
        vector<int> temp;
        dfs(candidates ,target ,0 ,candidates.size() ,temp);
        return res;
    }

    void dfs(vector<int>& candidates ,int target ,int start ,int end ,vector<int>& temp){
        if(target == 0){
            res.push_back(temp);
            return;
        }
        for(int i = start;i < end;i++){
            if(i > start && candidates[i] == candidates[i - 1])     continue;
            temp.push_back(candidates[i]);
            if(target - candidates[i] < 0){
                temp.pop_back();
                break;
            }
            dfs(candidates ,target - candidates[i] ,i + 1 ,end ,temp);
            temp.pop_back();
        }
    }
};

全排列

class Solution {
public:
    vector<vector<int>> ans; //答案
    vector<int> temp; //临时变量

    void DFS(vector<int>& nums, int n)
    {
        //出口
        if(n == nums.size())
        {
            ans.push_back(temp);
            return;
        }

        for(int i = 0; i < nums.size(); ++i)
        {
            //判断是否用过
            if(nums[i] >= -10)
            {
                temp.push_back(nums[i]);
                nums[i] -= 30; //数字范围为-10到10，所以我们可以将他变小来判断是否用过
                DFS(nums, n+1);
                nums[i] += 30; //复原
                temp.pop_back();
            }
        }
    }
    vector<vector<int>> permute(vector<int>& nums) {
        DFS(nums, 0);
        return ans;
    }
};

火柴拼正方形

class Solution {
public:
    vector<int> nums;
    vector<bool> st;
    bool makesquare(vector<int>& matchsticks) {
        nums = matchsticks;
        st.resize(nums.size());
        // 剪枝1：由大到小排列
        sort(nums.begin(), nums.end(), greater<int>());
        int sum = 0;
        for (auto x: nums)
            sum += x;
        if (sum % 4)
            return false;
        sum /= 4;
        return dfs(0, 0, sum, 0);
    }
    bool dfs(int u, int cur, int len, int cnt)
    {
        if (cnt == 3)
            return true;
        if (cur == len)
            return dfs(0, 0, len, cnt + 1);
        for (int i = u; i < nums.size(); i ++ )
        {
            if (st[i])
                continue;
            st[i] = true;
            if (dfs(i + 1, cur + nums[i], len, cnt))
                return true;
            st[i] = false;
             // 剪枝2：跳过长度相同的火柴
            while (i + 1 < nums.size() && nums[i + 1] == nums[i])
                i ++ ;
            // 剪枝3：如果当前是第一根或者最后一根火柴
            if (!cur || cur + nums[i] == len)
                return false;
        }
        return false;
    }
};