Update 0070.爬楼梯.md

添加 0070.爬楼梯 Java版本

problems/0070.爬楼梯.md

@@ -212,6 +212,22 @@ public:
 
 
 Java：
+```Java
+class Solution {
+    public int climbStairs(int n) {
+        // 跟斐波那契数列一样
+        if(n <= 2) return n;
+        int a = 1, b = 2, sum = 0;
+        
+        for(int i = 3; i <= n; i++){
+            sum = a + b;
+            a = b;
+            b = sum;
+        }
+        return b;
+    }
+}
+```
 
 
 Python：