原题网址：https://www.lintcode.com/problem/cosine-similarity/description
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]).
给出 A = [1, 2, 3], B = [2, 3 ,4].
返回 0.9926.
给出 A = [0], B = [0].
返回 2.0000
思路：直接按照公式计算就好。注意余弦相似度无效的情况，分母等于0或者空数组。
PS：一开始提交的代码没考虑到空数组以及数组全0的情况，只排除了样例中的无效情况……汗
AC代码：
class Solution {
public:
/*
* @param A: An integer array
* @param B: An integer array
* @return: Cosine similarity
*/
double cosineSimilarity(vector<int> &A, vector<int> &B) {
// write your code here
int n=A.size();
if (n==0)
{
return 2.0;
}
double x=0.0,sumA=0.0,sumB=0.0;
for (int i=0;i<n;i++)
{
x=x+A[i]*B[i];
sumA=sumA+A[i]*A[i];
sumB=sumB+B[i]*B[i];
}
sumA=sqrt(sumA);
sumB=sqrt(sumB);
if (sumA==0||sumB==0)
{
return 2.0;
}
double result=x/(sumA*sumB);
return result;
}
};
原文：https://www.cnblogs.com/Tang-tangt/p/9234436.html