Frog Jump

A frog is crossing a river. The river is divided into x 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.

Given a list of stones' positions (in units) in sorted ascending order, determine if the frog is able to cross the river by landing on the last stone. Initially, the frog is on the first stone and assume the first jump must be 1 unit.

If the frog's last jump was k units, then its next jump must be either k - 1, k, or k + 1 units. Note that the frog can only jump in the forward direction.

Note:

The number of stones is ≥ 2 and is < 1,100.
Each stone's position will be a non-negative integer < 231.
The first stone's position is always 0.

Example 1:

[0,1,3,5,6,8,12,17]

There are a total of 8 stones.
The first stone at the 0th unit, second stone at the 1st unit,
third stone at the 3rd unit, and so on...
The last stone at the 17th unit.

Return true. 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.

Example 2:

[0,1,2,3,4,8,9,11]

Return false. There is no way to jump to the last stone as 
the gap between the 5th and 6th stone is too large.

Solution 1:

BFS

class Solution {
    static class Node {
        int lastJump;
        int position;
        
        public Node(int position, int lastJump) {
            this.position = position;
            this.lastJump = lastJump;
        }
        
        @Override
        public int hashCode() {
            int hash = 17;
            hash += hash * 31 + this.position;
            hash += hash * 31 + this.lastJump;
            return hash;
        }
        
        @Override
        public boolean equals(Object o) {
            Node n = (Node) o;
            return n.position == this.position && n.lastJump == this.lastJump;
        }
    }
    
    public boolean canCross(int[] stones) {
        Queue<Node> queue = new ArrayDeque();
        Set<Integer> canVisit = new HashSet();
        Set<Node> visited = new HashSet();
        for (int s : stones) canVisit.add(s);
        Node start = new Node(0, 0);
        queue.offer(start);
        visited.add(start);
        while (!queue.isEmpty()) {
            Node curr = queue.poll();
            if (curr.position == stones[stones.length - 1]) {
                return true;
            }
            int lastJump = curr.lastJump;
            for (int nextJump = lastJump - 1; nextJump <= lastJump + 1; nextJump ++) {
                int nextPosition = curr.position + nextJump;
                Node next = new Node(nextPosition, nextJump);
                if (nextPosition > curr.position && canVisit.contains(nextPosition) && visited.add(next)) {
                    queue.offer(next);
                }
            }
        }
        return false;
    }
}

class Solution {
    public boolean canCross(int[] stones) {
        // dp[i][k] = from i can jump k units
        // dp[i][k +(-) 1] = true if dp[j][k] for stones[j] + k == stones[i]
        // dp[0][1] = true
        // any dp[n - 1][k]
        int n = stones.length;
        boolean[][] dp = new boolean[n][n + 1];
        dp[0][1] = true;
        for (int i = 1; i < n; i ++) {
            for (int j = 0; j < i; j ++) {
                int k = stones[i] - stones[j];
                if (k <= n && dp[j][k]) {
                    dp[i][k - 1] = true;
                    dp[i][k] = true;
                    dp[i][k + 1] = true;
                }
            }            
        }
        for (int k = 1; k <= n; k ++) {
            if (dp[n - 1][k]) {
                return true;
            }
        }
        return false;
    }
}