NC_150_BINARY_TREE_COUNT NC_151_GCD NC_156_FIND_ONCE_NUMBER

package org.example.interview.practice;

/**
 * @author xianzhe.ma
 * @date 2021/8/22
 * 左子树的数量乘以右子树的数量
 */

public class NC_150_BINARY_TREE_COUNT {

    private static int mod = 1000000007;
    public int numberOfTree (int n) {
        // 直接返回
        if (n == 1 || n == 2){
            return n;
        }
        long[] dp = new long[n+1];
        dp[0] = 1;
        dp[1] = 1;
        dp[2] = 2;
        for (int i = 3;i < dp.length;++i){
            long count = 0;
            for (int j = 1;j <= i;++j){
                count += ((dp[i-j]*dp[j-1])%mod);
            }
            dp[i] = (int) (count % mod);
        }

        return (int) dp[n];
    }
}

package org.example.interview.practice;

import java.util.regex.Pattern;

/**
 * @author xianzhe.ma
 * @date 2021/8/18
 */

public class NC_151_GCD {

    public static int gcd (int a, int b) {
        // write code here
        if (a%b == 0) {
            return b;
        } else {
            return gcd(b,a%b);
        }
    }

    public static void main (String[] args) {

        int a = 12;
        int b = 10;
        System.out.println(gcd(a,b));
    }

}

package org.example.interview.practice;

import org.redisson.misc.Hash;

import java.util.HashSet;
import java.util.Set;

/**
 * @author xianzhe.ma
 * @date 2021/7/14
 */

public class NC_156_FIND_ONCE_NUMBER {

    public static int foundOnceNumber (int[] arr, int k) {
        // write code here
        if (arr == null || arr.length == 0) {
            return -1;
        }
        Set set = new HashSet<>();
        Set nominate = new HashSet<>();

        int result = arr[0];
        for (int num : arr) {
            if (!set.contains(num)) {
                set.add(num);
                nominate.add(num);
            } else {
                nominate.remove(num);
            }
        }

        return nominate.stream().findFirst().get();
    }

    public static void main (String[] args) {
        int[] arr = {5,4,1,1,5,1,5};
        System.out.println(foundOnceNumber(arr, 3));
    }
}