add leetcode

Author: wave-gbt

algorithm/70. Climbing Stairs.md

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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
+```
+**分析**
+d(0) = 1
+
+d(1) = 1
+
+d(2) = d(2-1) + d(2-2)
+
+d(3) = d(3-1) + d(3-2)
+
+……
+Fibonacci数列,定义如下
+
+`f(1)=1,f(2)=1,f(n)=f(n-1)+f(n-2),n>2`
+
+**代码**
+```
+/**
+ *  Fibonacci数列,定义如下
+ *  f(1)=1,f(2)=1,f(n)=f(n-1)+f(n-2),n>2
+ *
+ **/
+
+class Solution {
+    public int climbStairs(int n) {
+        if (n<=1)
+             return 1;
+  
+          int oneStep=1,twoStep = 1,res = 0;
+  
+          for (int i = 2; i <= n; i++) {
+              res = oneStep + twoStep;
+             twoStep = oneStep;
+             oneStep = res;
+         }        
+         return res;
+    }
+}
+```