basics

Author: idf

src/ClimbingStairs/Solution.java

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+package ClimbingStairs;
+
+/**
+ * User: Danyang
+ * Date: 1/22/2015
+ * Time: 20:38
+ *
+ * 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?
+ */
+public class Solution {
+    /**
+     * basic dp
+     * f[i] = f[i-2]+f[i-1]
+     * @param n
+     * @return
+     */
+    public int climbStairs(int n) {
+        if(n<0)
+            return 0;
+        if(n==1)
+            return 1;
+        if(n==2)
+            return 2;
+
+        int a = 1;
+        int b = 2;
+        while(n>=3) {
+            int t = b;
+            b = a+b;
+            a = t;
+            n--;
+        }
+        return b;
+    }
+}

src/PowXN/Solution.java

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+package PowXN;
+
+/**
+ * User: Danyang
+ * Date: 1/22/2015
+ * Time: 20:11
+ *
+ * Implement pow(x, n).
+ */
+public class Solution {
+    /**
+     * classical quick power algorithm
+     * Notice
+     * 1. walk through the steps of algorithm
+     * 2. x<0 case, n<0 case
+     * @param x
+     * @param n
+     * @return
+     */
+    public double pow(double x, int n) {
+        if(n==0)
+            return 1;
+
+        int sign = x>0 || (n&1)==0 ? 1: -1;  // common bugs
+        boolean inverted = n <= 0;
+        x = Math.abs(x);
+        n = Math.abs(n);
+
+
+        double ret = 1;
+        double fac = x;
+        while(n>1) {
+            if((n&1)==1)
+                ret *= fac;
+            fac *= fac;
+            n /= 2;
+        }
+        ret *= fac;
+        ret *= sign;
+        if(inverted)
+            ret = 1/ret;
+        return ret;
+    }
+}