[leetcode][DP] Climbing Stairs

题目：

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?

分析：f(n) = f(n-1)+f(n-2)

class Solution {public:    int climbStairs(int n) {        if(n <= 2) return n;        int *table = new int[n+1];        table[0] = 0;        table[1] = 1;        table[2] = 2;        for(int i = 3; i < n+1; ++i){            table[i] = table[i-1] + table[i-2];        }        int res = table[n];        delete []table;        return res;    }};

优化空间效率：

class Solution {public:    int climbStairs(int n) {if (n < 3) return n;int minus1 = 2, minus2 = 1;int res;for (int i = 3; i <= n; ++i){res = minus2 + minus1;minus2 = minus1;minus1 = res;}return res;    }};