my two ways to solve this question


  • 0
    J
    /*
    1.state  f[i] the num to reach no i
    2.function f[i] = f[i-1]+ f[i-2]
    3.init: f[0] = 1;f[1] = 1;
    4.answer:f[n]
    
    time  O(n)   sapce  O(1)
     */
    public int climbStairs(int n) {
        if (n < 0) return 0;
        if (n <= 1) {
            return 1;
        }
    
        //1.state
        int[] dp = new int[n + 1];
    
        //3.init
        dp[0] = 1;
        dp[1] = 1;
    
        //2  function
        for (int i = 2; i <= n; i++) {
            dp[i] = dp[i - 1] + dp[i - 2];
        }
    
        //4.answer
        return dp[n];
    }
    
    //time O(n)   space O(1)
    public int climbStairs2(int n) {
        if (n < 0) return 0;
        if (n <= 1) {
            return 1;
        }
    
    
        int prev1 = 1;
        int prev2 = 1;
        int res = 0;
        for (int i = 2; i <= n; i++) {
            res = prev1 + prev2;
            prev1 = prev2;
            prev2 = res;
        }
        return res;
    }

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