刷题时用到的技巧

　　 1、使用memset进行初始化 memset(a,0,sizeof(a)); memset(a,-1,sizeof(a));

        2、合理使用sort函数，sort(a,a+n),sort(v.begin(),v.end());

        3、 取中值需要小心越界，int mid = l + ((r - l) >> 1);

　　 4、vector删除最一个pop_back

　　 5、vector<pair<int,int>>;

　　 6、vector声明一个二位数值 vector<vecotr<int>> vc(n,vector<int>(m));

        7、reverse 反转列表

　　 8、字符转字符串 string(1,char)　　，使用string构造

        9、substr(pos,count);

 

0034

在排序数组中查找元素的第一个和最后一个位置

需要结束的位置的时候，使用二分法需要  iMid = (iEnd + iHiegh + 1) / 2; 不加1会死循环

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        int iLow = 0;
        int iHiegh = nums.size()-1;
        int iK = -1;
        int iStart = -1,iEnd = -1;
        while(iLow <= iHiegh)
        {
            int iMid = (iLow + iHiegh) / 2;
            if(nums[iMid] > target)
            {
                iHiegh = iMid - 1;
            }
            else if(nums[iMid] < target)
            {
                iLow = iMid + 1;
            }
            else
            {
                iStart = iMid;
                iEnd = iMid;
                while(iLow < iStart)
                {
                    iMid = (iLow + iStart) / 2;
                    if(nums[iMid] != nums[iStart])
                    {
                        iLow = iMid + 1;
                    }
                    else
                    {
                        iStart = iMid;
                    }
                }
                while(iEnd < iHiegh)
                {
                    iMid = (iEnd + iHiegh + 1) / 2;    //这个地方不加1，会死循环
                    if(nums[iMid] != nums[iEnd])
                    {
                        iHiegh = iMid - 1;
                    }
                    else
                    {
                        iEnd = iMid;
                    }
                }
                break;
            }
        }


        return vector<int>{iStart,iEnd};

    }
};
View Code

 

0038 外观数列

不知道为啥 itoa 不能用，只能用 to_string()

class Solution {
private:
    string GetNextString(const string& str)
    {
        string strRes = "";
        int iLen = str.length();
        for(int i=0;i<iLen;)
        {
            char c = str[i];
            int j = 1;
            while(i+j < iLen)
            {
                if(str[i+j] != c)
                {
                    break;
                }
                ++j;
            }
            i += j;
            strRes += to_string(j); //itoa,居然会报错
            strRes += c;
        }
        return strRes;
    }
public:
    string countAndSay(int n) {
        string str = "1";

        for(int i=1;i<n;++i)
        {
            str = GetNextString(str);    
        }

        return str;
    }
};
View Code

 

 0040 组合总和

class Solution {
    vector<int> vcTemp;
    vector<vector<int>> vcRes;
    void dfs(vector<pair<int,int>>& vcpair,int target,int cur,const int& n)
    {
        if(target==0)
        {
            vcRes.push_back(vcTemp);
            return;
        }
        if(cur >= n || target < 0)
        {
            return;
        }
        dfs(vcpair,target,cur+1,n);
        int max = min(target / vcpair[cur].first,vcpair[cur].second);
        for(int i=1;i<=max;++i)
        {
            vcTemp.push_back(vcpair[cur].first);
            dfs(vcpair,target-vcpair[cur].first * i,cur+1,n);
        }
        for(int i=1;i<=max;++i)
        {
            vcTemp.pop_back();
        }
    }
public:
    vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {
       sort(candidates.begin(),candidates.end());
       vector<pair<int,int>> vcpair{(31,1)}; 
       for(int i=candidates.size()-1;i>=0;--i)
       {
           if(vcpair.back().first != candidates[i])
           {
               vcpair.push_back(pair<int,int>(candidates[i],1));
           }
           else
           {
               ++vcpair.back().second;
           }
       }
       int n = vcpair.size();
        dfs(vcpair,target,1,n);
       return vcRes;

    }
};
View Code

 

 

0046 全排列

C++中有函数 next_permutation

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        vector<vector<int>> vcRes;
        sort(nums.begin(),nums.end());
        do
        {
            vcRes.push_back(nums);
        }while(next_permutation(nums.begin(),nums.end()));
        return vcRes;
    }
};
View Code

 或者是DFS

class Solution {
private:
    void dfs(vector<int>& nums,int iCur,vector<int>& res)
    {
        if(iCur == m_iLen)
        {
            m_vcRes.push_back(res);
        }
        
        for(int i=0;i<m_iLen;++i)
        {
            if(!Hash[i])
            {
                res.push_back(nums[i]);
                Hash[i] = 1;
                dfs(nums,iCur+1,res);
                res.pop_back();
                Hash[i] = 0;
            }
        }
    }
public:
    vector<vector<int>> permute(vector<int>& nums) {
        memset(Hash,0,sizeof(Hash));
        m_iLen = nums.size();
        vector<int> temp;
        dfs(nums,0,temp);
        return m_vcRes;
    }
private:
    vector<vector<int>> m_vcRes;
    int Hash[7];
    int m_iLen;
};
View Code

 

0047 全排列2

直接使用C++函数  next_permutation

class Solution {
public:
    vector<vector<int>> permuteUnique(vector<int>& nums) {
        sort(nums.begin(),nums.end());
        vector<vector<int>> vcRes;
        do
        {
            vcRes.push_back(nums);
        }while(next_permutation(nums.begin(),nums.end()));
        return vcRes;
    }
};
View Code

 

0050 

Pow(x, n)

可以考虑n的二进制表现形式

注意点：考虑n为负数，然后变成正数的时候越界情况。

class Solution {
public:
    double myPow(double x, int n) {
        int64_t N = n;
        x = N > 0 ? x : 1.0 / x;
        N = N > 0 ? N : -N;

        double res = 1;
        while(N)
        {
            if(N & 1)
            {
                res *= x;
            }
            x *= x;
            N >>= 1;
        }
        return res;
    }
};
View Code

 

0056 合并区间

注意用力 [[1,4],[2,3]]

class Solution {
public:
    vector<vector<int>> merge(vector<vector<int>>& intervals) {
        auto comp = [](vector<int>& a,vector<int>& b) -> bool
        {
            if(a[0] == b[0])
            {
                if(a[1] == b[1])
                    return false;
                return a[1] < b[1];
            }
            return a[0] < b[0];
        };
        sort(intervals.begin(),intervals.end(),comp);

        vector<vector<int>> res;
        //加入哨兵
        intervals.push_back(vector<int>{10001,10001});
        int len = intervals.size();
        vector<int> temp = intervals[0];
        for(int i=1;i<len;++i)
        {
            if(temp[1] < intervals[i][0])
            {
                res.push_back(temp);
                temp = intervals[i];
            }
            else
            {
                temp[1] = max(temp[1],intervals[i][1]);
            }
        }
        
        return res;
    }
};
View Code