70. Climbing Stairs (509 Fibonacci Number)

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.

Example 1:

Input: 2
Output: 2
Explanation: There are two ways to climb to the top.
1. 1 step + 1 step
2. 2 steps

Example 2:

Input: 3
Output: 3
Explanation: There are three ways to climb to the top.
1. 1 step + 1 step + 1 step
2. 1 step + 2 steps
3. 2 steps + 1 step
  • Simple DP
public int climbStairs(int n) {
    if(n <= 1) return n;
    int x = 1, y = 2;
    for(int i = 2; i < n; i++) {
        int z = x + y;
        x = y;
        y = z;
    }
    return y;
}