栈和队列都使用 Deque<XX> deque = new LinkedList(); deque.offerFirst(); deque.pollFirst(); deque.offerLast(); deque.pollLast();
switch(str){case"+":
stack.offerLast( right + left);break;case"-":
stack.offerLast( left - right);break;case"*":
stack.offerLast( right * left);break;case"/":
stack.offerLast( left / right);break;default:}
new ArrayList(list)里面可以是List<Integer> 类型 也可以是 char[]类型
classSolution{publicList<List<String>>partition(String s){List<List<String>> ret =newArrayList();List<String> list =newArrayList();char[] chars = s.toCharArray();process(chars,0, list, ret);return ret;}publicvoidprocess(char[] chars,int index,List<String> list,List<List<String>> ret){if(index == chars.length){
ret.add(newArrayList(list));return;}for(int i = index; i < chars.length; i ++){if(isP(chars, index, i)){
list.add(newString(Arrays.copyOfRange(chars, index, i +1)));process(chars, i +1, list, ret);
list.remove(list.size()-1);}}}publicbooleanisP(char[] chars,int i,int j){if(i == j)returntrue;while(i < j){if(chars[i]!= chars[j]){returnfalse;}
i ++;
j --;}returntrue;}}
LCR 060.前K个高频元素
classSolution{publicint[]topKFrequent(int[] nums,int k){HashMap<Integer,Integer> map =newHashMap();for(int i =0; i < nums.length; i ++){
map.put(nums[i],map.getOrDefault(nums[i],0)+1);}PriorityQueue<HashMap.Entry<Integer,Integer>> queue =newPriorityQueue<>((a, b)-> b.getValue()- a.getValue()//没有分号);
queue.addAll(map.entrySet());//注意技巧int[] ret =newint[k];for(int i =0; i < k; i ++){
ret[i]= queue.poll().getKey();}return ret;}}