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算逆序数可以O(n)完成的

Posted by nightelf at 2007-08-28 12:35:42 on Problem 1007 
int count_inver(char *str, int len)
{
        int i;  
        int cnt = 0;
        int a[4] = {0};
        for(i = len - 1; i >= 0; i--) {
                switch (str[i]) {
                        case 'A':
                                a[1]++; 
                                a[2]++; 
                                a[3]++; 
                                break;  
                        case 'C':
                                a[2]++; 
                                a[3]++; 
                                cnt += a[1];
                                break;  
                        case 'G':
                                a[3]++; 
                                cnt += a[2];
                                break;  
                        case 'T':
                                cnt += a[3];
                }
        }
        return cnt;
}
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