DeleteAndEarn.java

public class DeleteAndEarn {
    /**
         Given an array nums of integers, you can perform operations on the array.

         In each operation, you pick any nums[i] and delete it to earn nums[i] points. After, you must delete every element equal to nums[i] - 1 or nums[i] + 1.

         You start with 0 points. Return the maximum number of points you can earn by applying such operations.

         Example 1:
         Input: nums = [3, 4, 2]
         Output: 6
         Explanation:
         Delete 4 to earn 4 points, consequently 3 is also deleted.
         Then, delete 2 to earn 2 points. 6 total points are earned.
         Example 2:
         Input: nums = [2, 2, 3, 3, 3, 4]
         Output: 9
         Explanation:
         Delete 3 to earn 3 points, deleting both 2's and the 4.
         Then, delete 3 again to earn 3 points, and 3 again to earn 3 points.
         9 total points are earned.
         Note:

         The length of nums is at most 20000.
         Each element nums[i] is an integer in the range [1, 10000].
     */
    public int deleteAndEarn(int[] nums) {
        if (nums == null || nums.length == 0) return 0;

        int[] freq = new int[10000];
        for (int i : nums) {
            freq[i]++;
        }
        int maxEarn = 0;
        int prevMaxEarn = 0;
        for (int i = 1; i < freq.length; i++) {
            int curMaxEarn = Math.max(prevMaxEarn + freq[i] * i, maxEarn);
            prevMaxEarn = maxEarn;
            maxEarn = curMaxEarn;
        }
        return maxEarn;
    }
}