[LintCode] 445 Cosine Similarity 解题报告

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.

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]).


Example
Given A = [1, 2, 3], B = [2, 3 ,4].
Return 0.9926.
Given A = [0], B = [0].
Return 2.0000


Code

class Solution {
    /**
     * @param A: An integer array.
     * @param B: An integer array.
     * @return: Cosine similarity.
     */
    public double cosineSimilarity(int[] A, int[] B) {
        // write your code here
        if (A == null || B == null || A.length == 0 || B.length == 0) {
            return 2.0000;
        }
        
        double numerator = 0;
        double denominator = 0;
        double squareSumA = 0;
        double squareSumB = 0;
        
        for (int i = 0; i < A.length; i++) {
            numerator += A[i] * B[i];
            squareSumA += A[i] * A[i];
            squareSumB += B[i] * B[i];
        }
        
        if (squareSumA == 0 || squareSumB == 0) {
            return 2.0000;
        }
        
        denominator = Math.sqrt(squareSumA) * Math.sqrt(squareSumB);
        return numerator / denominator;
        
    }
}