src/main/java/com/fishercoder/solutions/_464.java

package com.fishercoder.solutions;

import java.util.HashMap;
import java.util.Map;

public class _464 {
    public static class Solution1 {
        /**
         * Credit: https://discuss.leetcode.com/topic/68896/java-solution-using-hashmap-with-detailed-explanation
         */

        Map<Integer, Boolean> map;
        boolean[] used;

        public boolean canIWin(int maxChoosableInteger, int desiredTotal) {
            int sum = (1 + maxChoosableInteger) * maxChoosableInteger / 2;
            if (sum < desiredTotal) {
                return false;
            }
            if (desiredTotal <= 0) {
                return true;
            }

            map = new HashMap();
            used = new boolean[maxChoosableInteger + 1];
            return helper(desiredTotal);
        }

        public boolean helper(int desiredTotal) {
            if (desiredTotal <= 0) {
                return false;
            }
            int key = format(used);
            if (!map.containsKey(key)) {
                // try every unchosen number as next step
                for (int i = 1; i < used.length; i++) {
                    if (!used[i]) {
                        used[i] = true;
                        // check whether this lead to a win (i.e. the other player lose)
                        if (!helper(desiredTotal - i)) {
                            map.put(key, true);
                            used[i] = false;
                            return true;
                        }
                        used[i] = false;
                    }
                }
                map.put(key, false);
            }
            return map.get(key);
        }

        // transfer boolean[] to an Integer
        public int format(boolean[] used) {
            int num = 0;
            for (boolean b : used) {
                num <<= 1;
                if (b) {
                    num |= 1;
                }
            }
            return num;
        }
    }
}