相机最小覆盖问题:https://leetcode.com/problems/binary-tree-cameras/

// 本题测试链接 : https://leetcode.com/problems/binary-tree-cameras/
public class Code02_MinCameraCover {

    public static class TreeNode {
        public int value;
        public TreeNode left;
        public TreeNode right;
    }

    public static int minCameraCover1(TreeNode root) {
        Info data = process1(root);
        return (int) Math.min(data.uncovered + 1, Math.min(data.coveredNoCamera, data.coveredHasCamera));
    }

    // 潜台词：x是头节点，x下方的点都被覆盖的情况下
    public static class Info {
        public long uncovered; // x没有被覆盖，x为头的树至少需要几个相机
        public long coveredNoCamera; // x被相机覆盖，但是x没相机，x为头的树至少需要几个相机
        public long coveredHasCamera; // x被相机覆盖了，并且x上放了相机，x为头的树至少需要几个相机

        public Info(long un, long no, long has) {
            uncovered = un;
            coveredNoCamera = no;
            coveredHasCamera = has;
        }
    }

    // 所有可能性都穷尽了
    public static Info process1(TreeNode X) {
        if (X == null) { // base case
            return new Info(Integer.MAX_VALUE, 0, Integer.MAX_VALUE);
        }

        Info left = process1(X.left);
        Info right = process1(X.right);
        // x uncovered x自己不被覆盖，x下方所有节点，都被覆盖
        // 左孩子： 左孩子没被覆盖，左孩子以下的点都被覆盖
        // 左孩子被覆盖但没相机，左孩子以下的点都被覆盖
        // 左孩子被覆盖也有相机，左孩子以下的点都被覆盖
        long uncovered = left.coveredNoCamera + right.coveredNoCamera;

        // x下方的点都被covered，x也被cover，但x上没相机
        long coveredNoCamera = Math.min(
                // 1)
                left.coveredHasCamera + right.coveredHasCamera,

                Math.min(
                        // 2)
                        left.coveredHasCamera + right.coveredNoCamera,

                        // 3)
                        left.coveredNoCamera + right.coveredHasCamera));

        
        
        
        // x下方的点都被covered，x也被cover，且x上有相机
        long coveredHasCamera = 
                Math.min(left.uncovered, Math.min(left.coveredNoCamera, left.coveredHasCamera))

                + Math.min(right.uncovered, Math.min(right.coveredNoCamera, right.coveredHasCamera))

                + 1;

        return new Info(uncovered, coveredNoCamera, coveredHasCamera);
    }

    public static int minCameraCover2(TreeNode root) {
        Data data = process2(root);
        return data.cameras + (data.status == Status.UNCOVERED ? 1 : 0);
    }

    // 以x为头，x下方的节点都是被covered，x自己的状况，分三种
    public static enum Status {
        UNCOVERED, COVERED_NO_CAMERA, COVERED_HAS_CAMERA
    }

    // 以x为头，x下方的节点都是被covered，得到的最优解中：
    // x是什么状态，在这种状态下，需要至少几个相机
    public static class Data {
        public Status status;
        public int cameras;

        public Data(Status status, int cameras) {
            this.status = status;
            this.cameras = cameras;
        }
    }

    public static Data process2(TreeNode X) {
        if (X == null) {
            return new Data(Status.COVERED_NO_CAMERA, 0);
        }
        Data left = process2(X.left);
        Data right = process2(X.right);
        int cameras = left.cameras + right.cameras;
        
        // 左、或右，哪怕有一个没覆盖
        if (left.status == Status.UNCOVERED || right.status == Status.UNCOVERED) {
            return new Data(Status.COVERED_HAS_CAMERA, cameras + 1);
        }

        // 左右孩子，不存在没被覆盖的情况
        if (left.status == Status.COVERED_HAS_CAMERA || right.status == Status.COVERED_HAS_CAMERA) {
            return new Data(Status.COVERED_NO_CAMERA, cameras);
        }
        // 左右孩子，不存在没被覆盖的情况，也都没有相机
        return new Data(Status.UNCOVERED, cameras);
    }

}

给定一个数组arr，
返回如果排序之后，相邻两数的最大差值

要求：时间复杂度O(N)

import java.util.Arrays;

public class Code03_MaxGap {

    public static int maxGap(int[] nums) {
        if (nums == null || nums.length < 2) {
            return 0;
        }
        int len = nums.length;
        int min = Integer.MAX_VALUE;
        int max = Integer.MIN_VALUE;
        for (int i = 0; i < len; i++) {
            min = Math.min(min, nums[i]);
            max = Math.max(max, nums[i]);
        }
        if (min == max) {
            return 0;
        }
        // 不止一种数，min~max一定有range,  len个数据，准备len+1个桶
        boolean[] hasNum = new boolean[len + 1]; // hasNum[i] i号桶是否进来过数字
        int[] maxs = new int[len + 1];  // maxs[i] i号桶收集的所有数字的最大值
        int[] mins = new int[len + 1];  // mins[i] i号桶收集的所有数字的最小值
        int bid = 0; // 桶号
        for (int i = 0; i < len; i++) {
            bid = bucket(nums[i], len, min, max);
            mins[bid] = hasNum[bid] ? Math.min(mins[bid], nums[i]) : nums[i];
            maxs[bid] = hasNum[bid] ? Math.max(maxs[bid], nums[i]) : nums[i];
            hasNum[bid] = true;
        }
        int res = 0;
        int lastMax = maxs[0]; // 上一个非空桶的最大值
        int i = 1;
        for (; i <= len; i++) {
            if (hasNum[i]) {
                res = Math.max(res, mins[i] - lastMax);
                lastMax = maxs[i];
            }
        }
        return res;
    }

    // 如果当前的数是num，整个范围是min~max，分成了len + 1份
    // 返回num该进第几号桶
    public static int bucket(long num, long len, long min, long max) {
        return (int) ((num - min) * len / (max - min));
    }

    // for test
    public static int comparator(int[] nums) {
        if (nums == null || nums.length < 2) {
            return 0;
        }
        Arrays.sort(nums);
        int gap = Integer.MIN_VALUE;
        for (int i = 1; i < nums.length; i++) {
            gap = Math.max(nums[i] - nums[i - 1], gap);
        }
        return gap;
    }

    // for test
    public static int[] generateRandomArray(int maxSize, int maxValue) {
        int[] arr = new int[(int) ((maxSize + 1) * Math.random())];
        for (int i = 0; i < arr.length; i++) {
            arr[i] = (int) ((maxValue + 1) * Math.random()) - (int) (maxValue * Math.random());
        }
        return arr;
    }

    // for test
    public static int[] copyArray(int[] arr) {
        if (arr == null) {
            return null;
        }
        int[] res = new int[arr.length];
        for (int i = 0; i < arr.length; i++) {
            res[i] = arr[i];
        }
        return res;
    }

    // for test
    public static void main(String[] args) {
        int testTime = 500000;
        int maxSize = 100;
        int maxValue = 100;
        boolean succeed = true;
        for (int i = 0; i < testTime; i++) {
            int[] arr1 = generateRandomArray(maxSize, maxValue);
            int[] arr2 = copyArray(arr1);
            if (maxGap(arr1) != comparator(arr2)) {
                succeed = false;
                break;
            }
        }
        System.out.println(succeed ? "Nice!" : "Fucking fucked!");
    }

}