<p>A frog is crossing a river. The river is divided into some number of units, and at each unit, there may or may not exist a stone. The frog can jump on a stone, but it must not jump into the water.</p>

<p>Given a list of <code>stones</code>&nbsp;positions (in units) in sorted <strong>ascending order</strong>, determine if the frog can cross the river by landing on the last stone. Initially, the frog is on the first stone and assumes the first jump must be <code>1</code> unit.</p>

<p>If the frog&#39;s last jump was <code>k</code> units, its next jump must be either <code>k - 1</code>, <code>k</code>, or <code>k + 1</code> units. The frog can only jump in the forward direction.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> stones = [0,1,3,5,6,8,12,17]
<strong>Output:</strong> true
<strong>Explanation:</strong> The frog can jump to the last stone by jumping 1 unit to the 2nd stone, then 2 units to the 3rd stone, then 2 units to the 4th stone, then 3 units to the 6th stone, 4 units to the 7th stone, and 5 units to the 8th stone.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> stones = [0,1,2,3,4,8,9,11]
<strong>Output:</strong> false
<strong>Explanation:</strong> There is no way to jump to the last stone as the gap between the 5th and 6th stone is too large.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>2 &lt;= stones.length &lt;= 2000</code></li>
	<li><code>0 &lt;= stones[i] &lt;= 2<sup>31</sup> - 1</code></li>
	<li><code>stones[0] == 0</code></li>
	<li><code>stones</code>&nbsp;is sorted in a strictly increasing order.</li>
</ul>

using ll = long long;
class Solution {
public:
    int dp[2005][10000];
    unordered_map<int,int> mpp;
    bool rec(int lvl, int k, vector<int> &nums){
        // pruning
        if(k <= 0)
            return false;

        // base case
        if(lvl >= nums.size()-1){
            return true;
        } 

        // cache check
        if(dp[lvl][k] != -1)
            return dp[lvl][k];

        // check
        bool res = false;
        if(mpp.find(nums[lvl]+k) != mpp.end()){
            res = rec(mpp[nums[lvl]+k], k, nums) || rec(mpp[nums[lvl]+k], k-1, nums) || rec(mpp[nums[lvl]+k], k+1, nums); 
        }

        // result
        return dp[lvl][k] = res;

    }
    bool canCross(vector<int>& stones) {
        ios_base::sync_with_stdio(0); 
        cout.tie(0); cin.tie(0);
        memset(dp,-1,sizeof(dp));
        for(int i = 0; i<stones.size(); i++){
            mpp[stones[i]] = i; 
        }
        return rec(0,1,stones);
    }
};
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