LintCode_Cosine Similarity

問題描述：Cosine Similarity

Cosine similarity is a measure of similarity between two vectors of an inner product space that measures the cosine of the angle between them. The cosine of 0° is 1, and it is less than 1 for any other angle.

See wiki: Cosine Similarity

Here is the formula:

Given two vectors A and B with the same size, calculate the cosine similarity.

Return 2.0000 if cosine similarity is invalid (for example A = [0] and B = [0]).

算法思想：關鍵是通過循環分別求出分子和分母：

    public static double cosineSimilarity(int[] A, int[] B) {
        // write your code here
        double result=0.0;
        
        //運算因子
        double a=0.0;
        double b=0.0;
        double c=0.0;
        
        if(A==null||B==null){
            return 0.0;
        }

        if(A.length!=B.length){
            return 0.0;
        }
        
        if(A.length==0||(A.length==1&&A[0]==0&&B[0]==0)){
            return 2.0;
        }
        
        for(int i=0;i<A.length;i++){
            
            a+=A[i]*B[i];
            b+=Math.pow(A[i], 2);//等同於b+=A[i]*A[i];
            c+=Math.pow(B[i], 2);//等同於c+=B[i]*B[i];
        }
        
        if(b==0||c==0){
            return 2.0;
        }
        
        result=a/(Math.sqrt(b)*Math.sqrt(c));
        
        return result;
    }