LC 740. Delete and Earn

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].

 

Runtime: 4852 ms, faster than 0.72% of C++ online submissions for Delete and Earn.

 

自以为做了一个区间DP，结果并不是这样做。

 

//
// Created by yuxi on 2019/1/22.
//

#include <vector>
#include <unordered_map>
#include <algorithm>
using namespace std;

class Solution {
public:
  unordered_map<int,int> mp;
  unordered_map<int,int> memo;
  int deleteAndEarn(vector<int>& nums) {
    if(nums.empty()) return 0;
    for(int x : nums) mp[x]++;
    vector<int> keys;
    for(auto it = mp.begin(); it != mp.end(); it++) keys.push_back(it->first);
    sort(keys.begin(),keys.end());
    vector<vector<int>> dp(keys.size(), vector<int>(keys.size(),0));
    for(int k=0; k<keys.size(); k++) {
      for(int i=0; i<keys.size(); i++) {
        int j = i - k;
        if(j < 0) continue;
        if(i == j) {
          dp[j][i] = keys[i] * mp[keys[i]];
          continue;
        }
        for(int l = j; l < i; l++) {
          if(keys[l] == keys[l+1]-1) {
            if(l+1 == i) dp[j][i] = max(dp[j][i], max(dp[j][l],dp[i][i]));
            else dp[j][i] = max(dp[j][i], dp[j][l]+dp[l+2][i]);
          }else {
            dp[j][i] = max(dp[j][i], dp[j][l]+dp[l+1][i]);
          }
        }
      }
    }
    return dp[0][keys.size()-1];
  }
};

 

 

 

Runtime: 8 ms, faster than 87.68% of C++ online submissions for Delete and Earn.

 

//
// Created by yuxi on 2019/1/22.
//

#include <vector>
#include <unordered_map>
#include <algorithm>
using namespace std;

class Solution {
public:
  unordered_map<int,int> mp;
  unordered_map<int,int> memo;
  int deleteAndEarn(vector<int>& nums) {
    if(nums.empty()) return 0;
    int maxval = 0;
    for(int x : nums) {
      maxval = max(maxval,x);
      mp[x]++;
    }
    vector<int> a(maxval+1, 0);
    for(auto it=mp.begin(); it != mp.end(); it++) {
      a[it->first] = it->second;
    }
    int prev = 0, cur = 0;
    for(int i=1; i<a.size(); i++) {
      cur = max(cur, i > 2 ? a[i] * i + a[i-2] : a[i] * i);
      a[i] = cur;
    }
    return cur;
  }
};