Path Memorizing DFS Java Solution O(n)


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    S

    The general DFS cause huge run time redundancy. I use a map to memorize the calculated results.

    public class Solution {
        public int climbStairs(int n) {
            // DFS
            Map<Integer, Integer> map = new HashMap<Integer, Integer>();
            map.put(0,1);
            map.put(1,1);
            return dfs(n, map);
        }
        
        private int dfs(int n, Map<Integer, Integer> map){
            // fast ending
            if(map.containsKey(n)) return map.get(n);
            // recursion
            int rslt = dfs(n-1, map) + dfs(n-2, map);
            map.put(n, rslt);
            return rslt;
        }
    }

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