[leetcode]70. Climbing Stairs(Java)

https://leetcode.com/problems/climbing-stairs/#/description

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?

Note: Given n will be a positive integer.

package go.jacob.day707;public class Demo3 {/* * 空间复杂度O(1); */public int climbStairs(int n) {if (n < 3)return n;int one_step_before = 2;int two_steps_before = 1;int all_ways = 0;for (int i = 2; i < n; i++) {all_ways = one_step_before + two_steps_before;two_steps_before = one_step_before;one_step_before = all_ways;}return all_ways;}/* * 空间复杂度O(n); */public int climbStairs_2(int n) {if (n < 3)return n;int[] res = new int[n + 1];res[1] = 1;res[2] = 2;for (int i = 3; i <= n; i++) {res[i] = res[i - 1] + res[i - 2];}return res[n];}/* * 方法一：递归 时间复杂度高 */public int climbStairs_1(int n) {if (n < 1)return 0;if (n == 1)return 1;if (n == 2)return 2;return climbStairs_1(n - 1) + climbStairs_1(n - 2);}}