70.Climbing Stairs

递归解决方法

public class Main {
	int sum = 0;
 
	public int climbStairs(int n) {
		if (n <= 0) {//
			sum++;
 
		} else if (n == 1) {
			sum++;
		} else {
			climbStairs(n - 1);//每一种结果都有两种选择 要么一步 要么两步
			climbStairs(n - 2);
		}
 
		return sum;
	}
	public static void main(String[]args) {
		Main m=new Main();
		System.out.println(m.climbStairs(2));
	}
}

非递归

public class Main {
	int sum = 0;
 
	public int climbStairs(int n) {
		if (n <= 1) {
			return 1;
		} else {
			int[] arr = new int[n];
			arr[0] = 1;
			arr[1] = 2;
			int i = 2;
			while (i < n && i>= 2) {
				arr[i ] = arr[i-1] + arr[i - 2];//每一个都是前两位之和 
				i++;
				
			}
			return arr[n - 1];
		}
	}
 
	public static void main(String[] args) {
		Main m = new Main();
		System.out.println(m.climbStairs(8));
	}
}

其他人解法也是大同小异

private int[] memo;
 
public int climbStairs(int n) {
    memo = new int[n + 1];
    Arrays.fill(memo, -1);
    return getClimbStairs(n);
}
 
private int getClimbStairs(int n) {
    if (n == 1) {
        return 1;
    }
 
    if (n == 2) {
        return 2;
    }
 
    if (memo[n] == -1) {
        memo[n] = getClimbStairs(n - 1) + getClimbStairs(n - 2);
    }
    return memo[n];
}
}

int climbStairs(int n){
 
int x1=0,x2=1;
int i;
int sum;
for(i=1;i<=n;i++)
{
    sum=x1+x2;
    x1=x2;
    x2=sum;
}
return sum;
}

哈希表 有趣

public class Solution {
 
int result;
HashMap<Integer, Integer> memo = new HashMap<Integer, Integer>();
 
public int climbStairs(int n) {
    if(n < 2) {
        return 1;
    }
    if(memo.containsKey(n)) {
        return memo.get(n);
    }
    result = climbStairs(n-1) + climbStairs(n-2);
    memo.put(n,result);
    return result;
}
}