Given a string `s` and an int `k`, return all unique substrings of `s` of size `k` with `k` distinct characters.\n\n**Example 1:**\n```\nInput: s = "abcabc", k = 3\nOutput: ["abc", "bca", "cab"]\n```\n\n**Example 2:**\n```\nInput: s = "abacab", k = 3\nOutput: ["bac", "cab"]\n```\n\n**Example 3:**\n```\nInput: s = "awaglknagawunagwkwagl", k = 4\nOutput: ["wagl", "aglk", "glkn", "lkna", "knag", "gawu", "awun", "wuna", "unag", "nagw", "agwk", "kwag"]\nExplanation: \nSubstrings in order are: "wagl", "aglk", "glkn", "lkna", "knag", "gawu", "awun", "wuna", "unag", "nagw", "agwk", "kwag", "wagl" \n"wagl" is repeated twice, but is included in the output once.\n```\n\nConstraints:\n* The input string consists of only lowercase English letters `[a-z]`\n* 0 \u2264 `k` \u2264 26\n\n<details> \n  <summary>Solution</summary>\n\nJava sliding window: https://leetcode.com/playground/LRBxfw5W\n</details>

Given a string s and an int k, return all unique substrings of s of size k with k distinct characters.\n\nExample 1:\n\nInput: s = "abcabc", k = 3\nOutput: ["ab