Number of Sub-arrays With Odd Sum

数学题，要注意的是子数组是连续取的。
做完看题解发现就是个前缀和，但是我因为一开始没注意到子数组是连续的，最后写出来的逻辑稍微复杂了一点点。On & O1

impl Solution {
    pub fn num_of_subarrays(arr: Vec<i32>) -> i32 {
        let mut j_total = 0;
        let mut o_total = 0;
        let mut res = 0;
        let mut o_num = 0;
        let MOD = 1000000007;
        for i in arr {
            if i % 2 == 1 {
                res += j_total * o_num + o_total + o_num + 1;
                res %= MOD;
                let j_total_buf = j_total;
                j_total = o_total + o_num + 1;
                o_total = j_total_buf;
                o_num = 0;
            } else {
                o_num += 1;
            }
        }
        res + j_total * o_num
    }
}