164. Maximum Gap

Given an unsorted array, find the maximum difference between the successive elements in its sorted form.
Try to solve it in linear time/space.
Return 0 if the array contains less than 2 elements.
You may assume all elements in the array are non-negative integers and fit in the 32-bit signed integer range.

reference: https://leetcode.com/problems/maximum-gap/solution/

Solution：Bucket Sort

思路： 把n个数放到x个桶中。构造一个合适的x，max gap只可能出现在两个桶之间，不能出现在一个桶的内部。
Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

public class Solution {
    public int maximumGap(int[] num) {
        if (num == null || num.length < 2)
            return 0;
        // get the max and min value of the array
        int min = num[0];
        int max = num[0];
        for (int i:num) {
            min = Math.min(min, i);
            max = Math.max(max, i);
        }
        // the minimum possibale gap, ceiling of the integer division
        int gap = (int)Math.ceil((double)(max - min)/(num.length - 1));
        int[] bucketsMIN = new int[num.length - 1]; // store the min value in that bucket
        int[] bucketsMAX = new int[num.length - 1]; // store the max value in that bucket
        Arrays.fill(bucketsMIN, Integer.MAX_VALUE);
        Arrays.fill(bucketsMAX, Integer.MIN_VALUE);
        // put numbers into buckets
        for (int i:num) {
            if (i == min || i == max)
                continue;
            int idx = (i - min) / gap; // index of the right position in the buckets
            bucketsMIN[idx] = Math.min(i, bucketsMIN[idx]);
            bucketsMAX[idx] = Math.max(i, bucketsMAX[idx]);
        }
        // scan the buckets for the max gap
        int maxGap = Integer.MIN_VALUE;
        int previous = min;
        for (int i = 0; i < num.length - 1; i++) {
            if (bucketsMIN[i] == Integer.MAX_VALUE && bucketsMAX[i] == Integer.MIN_VALUE)
                // empty bucket
                continue;
            // min value minus the previous value is the current gap
            maxGap = Math.max(maxGap, bucketsMIN[i] - previous);
            // update previous bucket value
            previous = bucketsMAX[i];
        }
        maxGap = Math.max(maxGap, max - previous); // updata the final max value gap
        return maxGap;
    }
}