39. Combined sum

Realization

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       class 
       
       Solution {
       
       
    
       
       public 
       
       List
       
       <
       
       List
       
       <
       
       Integer
       
       >> 
       
       combinationSum(
       
       int[] 
       
       candidates, 
       
       int 
       
       target) {
       
       
        
       
       int 
       
       len 
       
       = 
       
       candidates.
       
       length;
       
       
        
       
       List
       
       <
       
       List
       
       <
       
       Integer
       
       >> 
       
       res 
       
       = 
       
       new 
       
       ArrayList
       
       <>();
       
       
        
       
       if (
       
       len 
       
       == 
       
       0) {
       
       
            
       
       return 
       
       res;
       
       
        }
       
       

       
       
        
       
       Deque
       
       <
       
       Integer
       
       > 
       
       path 
       
       = 
       
       new 
       
       ArrayDeque
       
       <>();
       
       
        
       
       dfs(
       
       candidates, 
       
       0, 
       
       len, 
       
       target, 
       
       path, 
       
       res);
       
       
        
       
       return 
       
       res;
       
       
    }
       
       

       
       
    
       
       /**
       
       
     
       
       * @param candidates  Candidate array 
       
       
     
       
       * @param begin  Search starting point 
       
       
     
       
       * @param len  Redundant variables , yes  candidates  In the properties of the , Can not pass 
       
       
     
       
       * @param target  Every element subtracted , The target value becomes smaller 
       
       
     
       
       * @param path  Path from root node to leaf node , It's a stack 
       
       
     
       
       * @param res  Result set list 
       
       
     
       
       */
       
       
    
       
       private 
       
       void 
       
       dfs(
       
       int[] 
       
       candidates, 
       
       int 
       
       begin, 
       
       int 
       
       len, 
       
       int 
       
       target, 
       
       Deque
       
       <
       
       Integer
       
       > 
       
       path, 
       
       List
       
       <
       
       List
       
       <
       
       Integer
       
       >> 
       
       res) {
       
       
        
       
       // target  It's a negative number and  0  No new children will be born when 
       
       
        
       
       if (
       
       target 
       
       < 
       
       0) {
       
       
            
       
       return;
       
       
        }
       
       
        
       
       if (
       
       target 
       
       == 
       
       0) {
       
       
            
       
       res.
       
       add(
       
       new 
       
       ArrayList
       
       <>(
       
       path));
       
       
            
       
       return;
       
       
        }
       
       

       
       
        
       
       //  Focus on understanding here from  begin  The meaning of starting the search 
       
       
        
       
       for (
       
       int 
       
       i 
       
       = 
       
       begin; 
       
       i 
       
       < 
       
       len; 
       
       i
       
       ++) {
       
       
            
       
       path.
       
       addLast(
       
       candidates[
       
       i]);
       
       

       
       
            
       
       //  Be careful ： Because each element can be reused , The starting point for the next round of search is still  i, It's very easy to make mistakes here 
       
       
            
       
       dfs(
       
       candidates, 
       
       i, 
       
       len, 
       
       target 
       
       - 
       
       candidates[
       
       i], 
       
       path, 
       
       res);
       
       

       
       
            
       
       //  Status reset 
       
       
            
       
       path.
       
       removeLast();
       
       
        }
       
       
    }
       
       

       
       
}
      
      
      
      

       
       
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44.
      
      

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