70. Climbing Stairs Java Solutin

You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

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public class Solution {
    public int climbStairs(int n) {
        if(n==1) return 1;
        if(n==2) return 2;
        return climbStairs(n-1) + climbStairs(n-2);
    }
}

递归，结果超时。

遂动态规划。

public class Solution {
    public int climbStairs(int n) {
         if(n < 4) return n;
        int a = 2, b = 3, c=5;
        for(int i = 4; i <n;i++){
            c = a + b;
            a = c - a;
            b = c;
            c = a + b;
        }
        return c;
    }
}