400_Nth_Digit and 463_Island_Perimeter

Author: qiyuangong

README.md

@@ -118,6 +118,7 @@ Remember solutions are only solutions to given problems. If you want full study
 | 387 | [First Unique Character in a String](https://leetcode.com/problems/first-unique-character-in-a-string/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/387_First_Unique_Character_in_a_String.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/387_First_Unique_Character_in_a_String.java) | Get frequency of each letter, return first letter with frequency 1, O(n) and O(1)  |
 | 388 | [Longest Absolute File Path](https://leetcode.com/problems/longest-absolute-file-path/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/388_Longest_Absolute_File_Path.py) | Store last length and rindex, O(n) and O(n) |
 | 389 | [Find the Difference](https://leetcode.com/problems/find-the-difference/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/389_Find_the_Difference.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/389_Find_the_Difference.java) | 1. Imaging letter a as 0, then the sum(t)-sum(s) is the result<br> 2. Based on solution 1, bit manipulate |
+| 400 | [Nth Digit](https://leetcode.com/problems/nth-digit/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/400_Nth_Digit.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/400_Nth_Digit.java) | islands * 4 - overlaps * 2  |
 | 401 | [Binary Watch](https://leetcode.com/problems/binary-watch/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/401_Binary_Watch.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/401_Binary_Watch.java) | Note that 12 * 60 is much less than 2^n or n^2. |
 | 404 | [Sum of Left Leaves](https://leetcode.com/problems/sum-of-left-leaves/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/404_Sum_of_Left_Leaves.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/404_Sum_of_Left_Leaves.java) | 1. Recursively DFS with root.left.left and root.left.right check<br>2. The same DFS based on stack |
 | 405 | [Convert a Number to Hexadecimal](https://leetcode.com/problems/convert-a-number-to-hexadecimal/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/405_Convert_a_Number_to_Hexadecimal.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/405_Convert_a_Number_to_Hexadecimal.java) | [Two's complement](https://en.wikipedia.org/wiki/Two%27s_complement) 1. Bit manipulate, each time handle 4 digits<br>2. Python (hex) and Java API (toHexString & Integer.toHexString) |
@@ -126,6 +127,7 @@ Remember solutions are only solutions to given problems. If you want full study
 | 416 | [Partition Equal Subset Sum](https://leetcode.com/problems/partition-equal-subset-sum/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/416_Partition_Equal_Subset_Sum.py) | DP, Check if sum of some elements can be half of total sum, O(total_sum / 2 * n) and O(total_sum / 2) |
 | 421 | [Maximum XOR of Two Numbers in an Array](https://leetcode.com/problems/maximum-xor-of-two-numbers-in-an-array/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/421_Maximum_XOR_of_Two_Numbers_in_an_Array.py) | Check 0~32 prefix, check if there is x y in prefixes, where x ^ y = answer ^ 1, O(32n) and O(n) |
 | 422 | [Valid Word Square](https://leetcode.com/problems/valid-word-square/) &hearts; | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/422_Valid_Word_Square.py) | Compare row with column, O(n^2) |
+| 463 | [Island Perimeter](https://leetcode.com/problems/island-perimeter/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/463_Island_Perimeter.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/463_Island_Perimeter.java) | math, find the area, actual number, then find the digit |
 
 | # | To Understand |
 |---| ----- |

java/400_Nth_Digit.java

@@ -0,0 +1,17 @@
+class Solution {
+    public int islandPerimeter(int[][] grid) {
+        // https://leetcode.com/problems/island-perimeter/discuss/95001/clear-and-easy-java-solution
+        int islands = 0, neighbours = 0;
+        for (int i = 0; i < grid.length; i++) {
+            for (int j = 0; j < grid[i].length; j++) {
+                if (grid[i][j] == 1) {
+                    islands++; // count islands
+                    if (i < grid.length - 1 && grid[i + 1][j] == 1) neighbours++; // count down neighbours
+                    if (j < grid[i].length - 1 && grid[i][j + 1] == 1) neighbours++; // count right neighbours
+                }
+            }
+        }
+
+        return islands * 4 - neighbours * 2;
+    }
+}

java/463_Island_Perimeter.java

@@ -0,0 +1,18 @@
+class Solution {
+    public int findNthDigit(int n) {
+		int len = 1;
+		long count = 9;
+		int start = 1;
+        // https://leetcode.com/problems/nth-digit/discuss/88363/Java-solution
+		while (n > len * count) {
+			n -= len * count;
+			len += 1;
+			count *= 10;
+			start *= 10;
+		}
+
+		start += (n - 1) / len;
+		String s = Integer.toString(start);
+		return Character.getNumericValue(s.charAt((n - 1) % len));
+	}
+}

python/400_Nth_Digit.py

@@ -0,0 +1,22 @@
+class Solution(object):
+    def islandPerimeter(self, grid):
+        """
+        :type grid: List[List[int]]
+        :rtype: int
+        """
+        # https://leetcode.com/problems/island-perimeter/discuss/95001/clear-and-easy-java-solution
+        row_num = len(grid)
+        if row_num == 0 || len(grid[0]) == 0:
+            return 0
+        islands, overlaps = 0, 0
+        col_num = len(grid[0])
+        for i in range(row_num):
+            for j in range(col_num):
+                if (grid[i][j] == 1):
+                    islands += 1
+                    # careful about right and down
+                    if (i < row_num - 1 && grid[i + 1][j] == 1):
+                        overlaps += 1
+                    if (j < col_num - 1 && grid[i][j + 1] == 1):
+                        overlaps += 1
+        return islands * 4 - overlaps * 2

python/463_Island_Perimeter.py

@@ -0,0 +1,20 @@
+class Solution(object):
+    def findNthDigit(self, n):
+        """
+        :type n: int
+        :rtype: int
+        """
+        # https://leetcode.com/problems/nth-digit/discuss/88363/Java-solution
+        count = 9
+        start = 1
+        curr_len = 1
+        while n > curr_len * count:
+            n -= curr_len * count
+            curr_len += 1
+            count *= 10
+            start *= 10
+        start += (n - 1) / curr_len
+        s = str(start)
+        return int(s[(n - 1) % curr_len]
+
+