70. 爬楼梯

70. 爬楼梯

动态规划

class Solution {
    public int climbStairs(int n) {
        if(n <= 2) return n;
        int[] dp = new int[n + 1];
        dp[1] = 1;
        dp[2] = 2;

        for(int i = 3; i <= n; i++){
            dp[i] = dp[i - 1] + dp[i - 2];
        }

        return dp[n];
    }
}

压缩空间

class Solution {
    public int climbStairs(int n) {
        if(n <= 2) return n;
        int a = 1;
        int b = 2;

        for(int i = 3; i <= n; i++){
            int c = a + b;
            a = b;
            b = c;
        }

        return b;
    }
}