LintCode: Climbing Stairs

C++

class Solution {
public:
    /**
     * @param n: An integer
     * @return: An integer
     */
    int climbStairs(int n) {
        // write your code here
        // f(0) = 1; 
        // f(1) = 1; 
        // f(2) = 2;
        // f(n) = f(n-1) + f(n-2) n>2;
        if (n == 2) {
            return 2;
        }
        if (n == 0 || n == 1) {
            return 1;
        }
        int a=1, b=2;
        //int c;
        for (int i=3; i<=n; i++) {
            b = a + b;
            a = b - a;
            // c = a + b;
            // a = b;
            // b = c;
        }
        return b;
    }
};