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ClimbingStairs70.java
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/**
* 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
*
*/
public class ClimbingStairs70 {
public int climbStairs(int n) {
if (n == 1) return 1;
int[] dp = new int[n+1];
dp[0] = 1;
dp[1] = 1;
for (int i=2; i<=n; i++) {
dp[i] = dp[i-1] + dp[i-2];
}
return dp[n];
}
public int climbStairs2(int n) {
if (n == 1) return 1;
int pre = 1;
int now = 1;
for (int i=2; i<=n; i++) {
int newVal = pre + now;
pre = now;
now = newVal;
}
return now;
}
}