LeetCode 1488.Avoid Flood in The City-Java-優先キュー

テーマリンク:1488.洪水の氾濫を避ける
Your country has an infinite number of lakes. Initially, all the lakes are empty, but when it rains over the nth lake, the nth lake becomes full of water. If it rains over a lake which is full of water, there will be a flood. Your goal is to avoid the flood in any lake.
Given an integer array rains where:

rains[i] > 0 means there will be rains over the rains[i] lake.

rains[i] == 0 means there are no rains this day and you can choose one lake this day and dry it.

Return an array ans where:

ans.length == rains.length

ans[i] == -1 if rains[i] > 0.

ans[i] is the lake you choose to dry in the ith day if rains[i] == 0.

If there are multiple valid answers return any of them. If it is impossible to avoid flood return an empty array.
Notice that if you chose to dry a full lake, it becomes empty, but if you chose to dry an empty lake, nothing changes. (see example 4)
Example 1:
Input: rains = [1,2,3,4] Output: [-1,-1,-1,-1] Explanation: After the first day full lakes are [1] After the second day full lakes are [1,2] After the third day full lakes are [1,2,3] After the fourth day full lakes are [1,2,3,4] There’s no day to dry any lake and there is no flood in any lake.
Example 2:
Input: rains = [1,2,0,0,2,1] Output: [-1,-1,2,1,-1,-1] Explanation: After the first day full lakes are [1] After the second day full lakes are [1,2] After the third day, we dry lake 2. Full lakes are [1] After the fourth day, we dry lake 1. There is no full lakes. After the fifth day, full lakes are [2]. After the sixth day, full lakes are [1,2]. It is easy that this scenario is flood-free. [-1,-1,1,2,-1,-1] is another acceptable scenario.
Example 3:
Input: rains = [1,2,0,1,2] Output: [] Explanation: After the second day, full lakes are [1,2]. We have to dry one lake in the third day. After that, it will rain over lakes [1,2]. It’s easy to prove that no matter which lake you choose to dry in the 3rd day, the other one will flood.
Example 4:
Input: rains = [69,0,0,0,69] Output: [-1,69,1,1,-1] Explanation: Any solution on one of the forms [-1,69,x,y,-1], [-1,x,69,y,-1] or [-1,x,y,69,-1] is acceptable where 1 <= x,y <= 10^9
Example 5:
Input: rains = [10,20,20] Output: [] Explanation: It will rain over lake 20 two consecutive days. There is no chance to dry any lake.
Constraints:

1 <= rains.length <= 10^5

0 <= rains[i] <= 10^9

問題解
コードを考慮せずに、直接問題に直面して、この結論を出すことができます.できるだけ先に洪水が発生する可能性が高い湖を乾かします.どんな湖が最初に洪水を起こすのか、もちろん水がいっぱい入っていて、将来最初に雨が降りやすい洪水です.これらの湖の隠れた危険を先に排除すれば解決するのではないでしょうか.
優先キューを使用する必要があります.ここではPriorityQueueで実装します.
Javaコード

/**
 * 
    ：2020-06-21 12:35:10
 */
class Solution {
    public int[] avoidFlood(int[] rains) {
        //   
        int days = rains.length;
        Map<Integer, Integer> map = new HashMap<>(days);
        // next[i]=k   rains[i]          k 
        int[] next = new int[days];
        //            
        for (int i = days - 1; i >= 0; i--) {
            //   rains[i]           next   
            next[i] = map.getOrDefault(rains[i], days);
            //    rains[i]            
            map.put(rains[i], i);
        }

        int[] res = new int[days];
        Arrays.fill(res, -1);
        //         ，             ，         ，        ，    
        Queue<Lake> full = new PriorityQueue<>(days, Comparator.comparingInt(lake -> lake.nextDay));
        //            
        for (int i = 0; i < days; i++) {
            if (rains[i] == 0) {
                //       
                if (full.isEmpty()) {
                    //        
                    res[i] = 1;
                } else {
                    //   k     ，k         
                    res[i] = full.poll().index;
                }
            } else {
                //  rains[i]   
                Lake lake = new Lake();
                //     
                lake.index = rains[i];
                //             
                lake.nextDay = next[i];
                //         
                full.offer(lake);
            }
            if (!full.isEmpty() && full.peek().nextDay <= i) {
                //                              ，    
                return new int[0];
            }
        }
        return res;
    }

    /**
     *   
     */
    private static class Lake {
        /**
         *     
         */
        int index;
        /**
         *             
         */
        int nextDay;
    }
}