Leetcode 题目解析之 Climbing Stairs

    public int climbStairs(int n) {
        if (n < 0) {
            return -1;
        } else if (n <= 2) {
            return n;
        }
        // 定义三个变量，空间复杂度是O(1)
        int step1 = 1;
        int step2 = 2;
        int step3 = 0;
        // 三个变量一直循环
        // climbStairs(n) = climbStairs(n - 1) + climbStairs(n - 2)
        for (int i = 3; i <= n; i++) {
            step3 = step1 + step2;
            step1 = step2;
            step2 = step3;
        }
        return step3;
    }