Map: HashMap, TreeMap

[YuezhenQin]

A Map is an object that maps keys to values, or is a collection of key-value pairs. It models the function abstraction in mathematics.

[YuezhenQin]

Anytime you need to count anything, think about using a hash map to do it.

1. k distinct chars: Find the length of the longest substring that contains at most k distinct characters.
2. top k: Find the k most frequent elements.

[YuezhenQin]

How to increment a HashMap value in Java?

map.put(map.getOrDefault(key, 0) + 1)

[YuezhenQin]

TreeMap

[YuezhenQin]

Find the Key associated with max Value in a Java Map

Map.Entry<Integer, Integer> maxEntry = null;

for (Map.Entry<Integer, Integer> entry : map.entrySet())
{
    if (maxEntry == null || entry.getValue().compareTo(maxEntry.getValue()) > 0)
    {
        maxEntry = entry;
    }
}