Map merge

将新的值赋值给map（如果不存在）或更新具有给定key的现有值。

        Map<Integer, Integer> map = new HashMap<>();
        for (Integer n : nums) {
            if(map.get(n) != null) {
                map.put(n, map.get(n)+1);
            } else {
                map.put(n, 1);
            }
        }
        Map<Integer, Integer> map = new HashMap<>();
        for (Integer n : nums) {
            map.merge(n, 1, Integer::sum);
        }

    default V merge(K key, V value,
            BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        Objects.requireNonNull(value);
        V oldValue = get(key);
        V newValue = (oldValue == null) ? value :
                   remappingFunction.apply(oldValue, value);
        if(newValue == null) {
            remove(key);
        } else {
            put(key, newValue);
        }
        return newValue;
    }