和为k的子数组

class Solution {
    public int subarraySum(int[] nums, int k) {
        int  count = 0;
        for(int i = 0; i < nums.length; i++){
            int sum = 0;
            for(int j = i; j >=0; j--){
                sum+= nums[j];
                if(sum == k){
                    count++;
                }
            }

        }
        return count;

    }
}

滑动窗口最大值

class Solution {
    public int[] maxSlidingWindow(int[] nums, int k) {
        if(nums.length == 0 || k == 0) return new int[0];
        Deque<Integer> deque = new LinkedList<>();
        int[] res = new int[nums.length - k + 1];
        for(int j = 0, i = 1 - k; j < nums.length; i++, j++) {
            // 删除 deque 中对应的 nums[i-1]
            if(i > 0 && deque.peekFirst() == nums[i - 1])
                deque.removeFirst();
            // 保持 deque 递减
            while(!deque.isEmpty() && deque.peekLast() < nums[j])
                deque.removeLast();
            deque.addLast(nums[j]);
            // 记录窗口最大值
            if(i >= 0)
                res[i] = deque.peekFirst();
        }
        return res;
    }
}

最小覆盖子串

class Solution {
    public String minWindow(String s, String t) {
        Map<Character,Integer> windows = new HashMap<>();
        Map<Character,Integer> need = new HashMap<>();
        for(int i = 0; i< t.length(); i++){
            need.put(t.charAt(i), need.getOrDefault(t.charAt(i), 0)+1);
        }
        int left = 0;
        int right = 0;
        int valdate = 0; 
        int start = 0;
        int len = Integer.MAX_VALUE;
        while(right < s.length()){
            char c = s.charAt(right);
            right++;
            if(need.containsKey(c)){
                windows.put(c, windows.getOrDefault(c, 0)+1);
                if(windows.get(c).equals(need.get(c))){
                    valdate++;
                }
            }
            while(valdate == need.size()){
                if(right- left < len){
                    start = left;
                    len = right-left;
                }
                char d = s.charAt(left);
                left++;
                if(need.containsKey(d)){
                    if(windows.get(d).equals(need.get(d))){
                        valdate--;
                    }
                    windows.put(d, windows.getOrDefault(d, 0)-1);
                }
            }
        }
        return len == Integer.MAX_VALUE? "": s.substring(start, start+len);

    }
}