70. Climbing Stairs

DP

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class Solution {
    public int climbStairs(int n) {
        // 定义 dp[i] 为爬至 i 台阶的方法数
        int[] dp = new int[n + 1];
        dp[0] = 1; // 注意爬至台阶 0 的方法数为 1 种
        dp[1] = 1;

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

        return dp[n];
    }
}

DP

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class Solution {
    public int climbStairs(int n) {
        int a = 1, b = 1;

        for (int i = 2; i <= n; i++) {
            int c = a + b;
            a = b;
            b = c;
        }

        return b;
    }
}

References

70. Climbing Stairs
剑指 Offer 10- II. 青蛙跳台阶问题