added Sort Colors (medium)

Author: minhyeong-joe

Medium/SortColors/README.md

@@ -0,0 +1,25 @@
+
+# Sort Colors
+[Leetcode Link](https://leetcode.com/problems/sort-colors/)
+
+## Problem:
+
+Given an array with `n` objects colored red, white or blue, sort them **in-place** so that objects of the same color are adjacent, with the colors in the order red, white and blue.
+
+Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.
+
+Note: You are not suppose to use the library's sort function for this problem.
+
+## Example:
+
+```
+Input: [2,0,2,1,1,0]
+Output: [0,0,1,1,2,2]
+```
+
+## Follow up:
+
+- A rather straight forward solution is a two-pass algorithm using counting sort.
+
+ First, iterate the array counting number of 0's, 1's, and 2's, then overwrite array with total number of 0's, then 1's and followed by 2's.
+- Could you come up with a one-pass algorithm using only constant space?

Medium/SortColors/Solution.java

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+package Medium.SortColors;
+
+import java.util.Arrays;
+
+/**
+ * Use 'low' to keep track of entry of next '0',
+ * use 'middle' to iterate over color array
+ * use 'high' to keep track of entry of next '2'
+ */
+public class Solution {
+
+ public void sortColors(int[] nums) {
+ int l = 0,
+ m = 0,
+ h = nums.length-1;
+ while (m <= h) {
+ switch (nums[m]) {
+ case 0:
+ swap(nums, l, m);
+ l++;
+ // no break equivalent to
+ // m++; break; (because it flows to case 1)
+ case 1:
+ m++;
+ break;
+ case 2:
+ swap(nums, m, h);
+ h--;
+ break;
+ }
+ }
+ }
+
+ // helper swap function
+ private void swap(int[] nums, int a, int b) {
+ int temp = nums[a];
+ nums[a] = nums[b];
+ nums[b] = temp;
+ }
+
+ // test driver
+ public static void main(String[] args) {
+ Solution sol = new Solution();
+ int[] nums = {2,0,2,1,1,0};
+ System.out.println("Input: " + Arrays.toString(nums));
+ sol.sortColors(nums);
+ System.out.println("Output: " + Arrays.toString(nums));
+ }
+
+}

README.md

@@ -74,6 +74,7 @@ Languages used: Java and Python
  - [Find Minimum in Rotated Sorted Array](Medium/FindMinInRotatedSortedArray)
  - [Maximum Swap](Medium/MaximumSwap)
  - [Number of Operations to Make Network Connected](Medium/NumToMakeNetworkConnected)
+ - [Sort Colors](Medium/SortColors)
 - Hard
  - [Maximum Score Words Formed by Letters](Hard/MaximumScoreWords)
  - [Reducing Dishes](Hard/ReducingDishes)