[Function add]

1. Add leetcode solutions.

Author: Seanforfun

README.md

@@ -431,6 +431,13 @@
 
 [258. Add Digits](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/258.%20Add%20Digits.md)
 
+[263. Ugly Number](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/263.%20Ugly%20Number.md)
+
+[264. Ugly Number II](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/264.%20Ugly%20Number%20II.md)
+
+[268. Missing Number](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/268.%20Missing%20Number.md)
+
+
 ## Algorithm(4th_Edition)
 Reading notes of book Algorithm(4th Algorithm),ISBN: 9787115293800.
 All java realization codes are placed in different packages.

leetcode/263. Ugly Number.md

@@ -39,4 +39,19 @@ class Solution {
         return false;
     }
 }
+```
+
+### Second time
+1. I still used the method of Recursion.
+```Java
+class Solution {
+    public boolean isUgly(int num) {
+        if(num == 1) return true;
+        if(num <= 0) return false;
+        if(num % 2 == 0) return isUgly(num / 2);
+        else if(num % 3 == 0) return isUgly(num / 3);
+        else if(num % 5 == 0) return isUgly(num / 5);
+        else return false;
+    }
+}
 ```

leetcode/264. Ugly Number II.md

@@ -31,4 +31,27 @@ class Solution {
         return false;
     }
 }
+```
+
+### Second time
+1. This question can be solved by dp
+2. except for 1, all the other values in dp array should be 2,3 or 5 multipling a previous value in the array.
+3. We hold three indices for 2, 3 and 5 so we can know what is the "current" value this value is multipling to.
+4. For each index in dp array, we have a change to select which numbers(may duplicate) by comparing the value, and easy to understand, dp[i] save the minimum number hold by (2, 3, 5).
+```Java
+class Solution {
+    public int nthUglyNumber(int n) {
+        int[] dp = new int[n + 1];
+        dp[0] = 1;
+        int index2 = 0, index3 = 0, index5 = 0;
+        int factor2 = 2, factor3 = 3, factor5 = 5;
+        for(int i = 1; i <= n; i++){
+            dp[i] = Math.min(factor2, Math.min(factor3, factor5));
+            if(dp[i] == factor2) factor2 = dp[++index2] * 2;
+            if(dp[i] == factor3) factor3 = dp[++index3] * 3;
+            if(dp[i] == factor5) factor5 = dp[++index5] * 5;
+        }
+        return dp[n - 1];
+    }
+}
 ```

leetcode/268. Missing Number.md

@@ -37,4 +37,19 @@ class Solution {
         return sum;
     }
 }
+```
+
+###Second time
+1. We first get the sum of the array and the missing value is the deduction between expect sum and sum.
+```Java
+class Solution {
+    public int missingNumber(int[] nums) {
+        int sum = 0;
+        for(int num : nums){
+            sum += num;
+        }
+        int expect = (nums.length) * (nums.length + 1) / 2;
+        return expect - sum;
+    }
+}
 ```