[LeetCode] 70. Climbing Stairs 爬楼梯

2021年09月15日 阅读数：6

You are climbing a stair case. It takes n steps to reach to the top.html

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?java

Note: Given n will be a positive integer.python

Example 1:数组

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

Example 2:post

```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```

state: dp[i] 表示爬到第i个楼梯的全部方法的和
function： dp[i] = dp[i-1] + dp[i-2]  //由于每次走一步或者两步， 因此dp[i]的方法就是它一步前和两步前方法加和
initial： dp[0] = 0; dp[1] = 1
end : return dp[n]htm

Java: Method 1: Time: O(n), Space: O(n)blog

```public int climbStairs(int n) {
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];
}
```

Java: Method 2: Time:  O(n), Space: O(1)three

```public int climbStairs(int n) {
if (n == 0 || n == 1 || n == 2){
return n;
}
int [] dp = new int[3];
dp[1] = 1;
dp[2] = 2;
for (int i =3; i <= n; i++) {
dp[i%3] = dp[(i-1)%3] + dp[(i-2)%3];
}
return dp[n%3];
}
```

Java: Method 3: Time:  O(n), Space: O(1)get

```public class Solution {
public int climbStairs(int n) {
int[] dp = new int[]{0,1,2};
if(n < 3) return dp[n];
for(int i = 2; i < n; i++){
dp[0] = dp[1];
dp[1] = dp[2];
dp[2] = dp[0] + dp[1];
}
return dp[2];
}
}
```

Java：

```public class Solution {
public int climbStairs(int n) {
if (n <= 1) return 1;
int[] dp = new int[n];
dp[0] = 1; dp[1] = 2;
for (int i = 2; i < n; ++i) {
dp[i] = dp[i - 1] + dp[i - 2];
}
return dp[n - 1];
}
}
```

Python: DP

```class Solution(object):
def climbStairs(self, n):
if n < 3:
return n
dp = [0] * n
dp[0] = 1
dp[1] = 2
for i in range(2, n):
dp[i] = dp[i-2] + dp[i-1]

return dp[n-1]　　```

Python:  DP, Time: O(n) Space: O(1)

```class Solution:
def climbStairs(self, n):
prev, current = 0, 1
for i in xrange(n):
prev, current = current, prev + current,
return current
```

Python:  Recursion，Time:  O(2^n) Space: O(n)

```class Solution:
def climbStairs1(self, n):
if n == 1:
return 1
if n == 2:
return 2
return self.climbStairs(n - 1) + self.climbStairs(n - 2)
```

C++:

```class Solution {
public:
int climbStairs(int n) {
if (n <= 1) return 1;
vector<int> dp(n);
dp[0] = 1; dp[1] = 2;
for (int i = 2; i < n; ++i) {
dp[i] = dp[i - 1] + dp[i - 2];
}
return dp.back();
}
};　　```

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