LeetCode 282. Expression Add Operators(DFS)

题目

题意：给你一个只有0-9组成的字符串，让你在其中插入'+'，'-'，'*'

使其计算得到的值恰好等于target，输出所有的可能性。

题解：不给数据范围，很显然就会考虑到超时，每个字符后面有四种选择，如果暴搜的话，字符长度超过10几个，就很快超时了。

但是我可以告诉你，这题目的字符串最多只有10个，所以不用考虑暴搜超时。这道题目考的是，你在暴搜到最后，计算这个表达式，会超时。

所以考你的就是在暴搜过程中，把表达式的值给计算出来。我个人而言，不喜欢这种不给数据范围，然后又不是正统套路的题目，。

class Solution {
public:
    vector<string> ans;
    int target2;
    vector<string> addOperators(string num, int target) {
        
        if(num.length()==0)
            return ans;
        
        target2 = target;
        
        string s="";
        s+=num[0];
        fun(num,s,0,-1,-1,num[0]-'0','.','.');
        
        return ans;
        
    }
    
    void fun(string num,string res,int pos,int last,int pre,int root,char operate,char operate2)
    {
        if(pos==num.length()-1)
        {
            if(operate!='.'&&operate2!='.')
            {
                if(operate2=='*')
                {
                    if(compute(root,compute(pre,last,'*'),operate)==target2)
                    {
                        ans.push_back(res);
                    }
                }
                else
                {
                    if(compute(compute(root,pre,operate),last,operate2)==target2)
                    {
                        ans.push_back(res);
                    }
                }
            }
            else if(operate!='.'&&operate2=='.')
            {
                if(compute(root,pre,operate)==target2)
                    ans.push_back(res);
            }
            else
            {
                if(root==target2)
                    ans.push_back(res);
            }
            return;
        }
     
        if(operate=='.')
        {
            if(root!=0&&((long long int)root)*10+(long long int)(num[pos+1]-'0') <= INT_MAX)
            {
                 fun(num,res+num[pos+1],pos+1,-1,-1,root*10+(num[pos+1]-'0'),'.','.');
            }

            fun(num,res+"+"+num[pos+1],pos+1,-1,num[pos+1]-'0',root,'+','.');
            
            fun(num,res+"-"+num[pos+1],pos+1,-1,num[pos+1]-'0',root,'-','.');
            
            fun(num,res+"*"+num[pos+1],pos+1,-1,num[pos+1]-'0',root,'*','.');
            
        }
        else if(operate!='.'&&operate2=='.')
        {
            if(pre!=0&&((long long int)pre)*10+(long long int)(num[pos+1]-'0') <= INT_MAX)
            {
            fun(num,res+num[pos+1],pos+1,-1,pre*10+(num[pos+1]-'0'),root,operate,'.');
            }

            fun(num,res+"+"+num[pos+1],pos+1,num[pos+1]-'0',pre,root,operate,'+');
            
            fun(num,res+"-"+num[pos+1],pos+1,num[pos+1]-'0',pre,root,operate,'-');
            
            fun(num,res+"*"+num[pos+1],pos+1,num[pos+1]-'0',pre,root,operate,'*');
            
        }
        else
        {
            if(last!=0&&((long long int)last)*10+(long long int)(num[pos+1]-'0') <= INT_MAX)
            {
            fun(num,res+num[pos+1],pos+1,last*10+(num[pos+1]-'0'),pre,root,operate,operate2);
            }
               
            if(operate=='*')
            {
                fun(num,res+"+"+num[pos+1],pos+1,num[pos+1]-'0',last,root*pre,operate2,'+');
                
                fun(num,res+"-"+num[pos+1],pos+1,num[pos+1]-'0',last,root*pre,operate2,'-');
                
                fun(num,res+"*"+num[pos+1],pos+1,num[pos+1]-'0',last,root*pre,operate2,'*');
            }
            else if(operate2=='*')
            {
                fun(num,res+"+"+num[pos+1],pos+1,num[pos+1]-'0',pre*last,root,operate,'+');
                
                fun(num,res+"-"+num[pos+1],pos+1,num[pos+1]-'0',pre*last,root,operate,'-');
                
                fun(num,res+"*"+num[pos+1],pos+1,num[pos+1]-'0',pre*last,root,operate,'*');
            }
            else
            {
               
                fun(num,res+"+"+num[pos+1],pos+1,num[pos+1]-'0',last,operate=='+'?root+pre:root-pre,operate2,'+');
                
                fun(num,res+"-"+num[pos+1],pos+1,num[pos+1]-'0',last,operate=='+'?root+pre:root-pre,operate2,'-');
                
                fun(num,res+"*"+num[pos+1],pos+1,num[pos+1]-'0',last,operate=='+'?root+pre:root-pre,operate2,'*');
                
            }
            
            
           
        }
         
    }
    
    int compute(int x,int y,char operate)
    {
        if(operate=='+')
            return x+y;
        else if(operate=='-')
            return x-y;
        else
        {
            if((long long int)x*(long long int)y>INT_MAX)
                return -1;
            return x*y;
        }
    }
    
};