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

解法1

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class Solution {
public:
    int climbStairs(int n) {
        if(n < 1){
            return 0;
        }
        vector<int> temp(n+2,0);
        int index = 0;
        temp[index++] = 1;
        temp[index++] = 1;
        while (index <= n) {
            temp[index] = temp[index -1 ]+temp[index-2];
            index++;
        }
        return temp[n];
    }
};

解法2

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class Solution {
public:
    int climbStairs(int n) {
        if(n < 1){
            return 0;
        }
        int a,b,c;
        a = 1;
        b = 0;
        c = a+b;
        int current = 1;
        while (current <= n) {
            a = b;
            b = c;
            c = a+b;
            current ++;
        }
        return c;
    }
};