在java中查找字符串中字符频率的有效方法:O(n)

In a recent interview I was asked to write the below program. Find out the character whose frequency is minimum in the given String ? So I tried by iterating through the string by using charAt and storing the character as key in a HashMap and the number of occurences as its value. Now Again I have to iterate on the Map to find the lowest element.In a recent interview I was asked to write the