Li Kou daily question - day 29 -523 Count odd numbers in interval range

2022.6.27 Did you brush the questions today

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subject ：

Here are two nonnegative integers  low  and  high . Please return  low  and  high  Between （ Including both ） An odd number .

analysis ：

Given an array , Find the number of odd numbers in this closed array , Note that you can take two numbers of array bounds .

Ideas ： Let's go through this array , Returns the value when the element is odd ++, The final result .

analysis ：

1. Violent solution

class Solution {
public:
    int countOdds(int low, int high) {
        int res = 0;
        for (int i = low; i <= high; i++)
        {
            if (i % 2 != 0)
            {
                res++;
            }
        }
        return res;
        
        
    }
};

2. The mathematical formula

because 【0-x】 Number of odd numbers in the interval prex（x）=（（x+1）÷2）. therefore 【x,y】 In the interval, we can use prex（y）-prex（x） Get

class Solution {
public:
    int pre(int x)
    {
        return (x + 1) /2;
    }
    int countOdds(int low, int high) {
        return pre(high) - pre(low-1);
    }
};