Create 2014-longest-subsequence-repeated-k-times.js

Author: everthis

2014-longest-subsequence-repeated-k-times.js

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+/**
+ * @param {string} s
+ * @param {number} k
+ * @return {string}
+ */
+var longestSubsequenceRepeatedK = function(s, k) {
+    // Max length of the subsequence can be determined by the length of the string and k
+    const maxLen = Math.floor(s.length / k);
+
+    // Find all possible characters that can appear in the subsequence (characters must appear at
+    // least k times in s)
+    const charCount = new Map();
+    const possibleChars = []
+
+    for (const char of s) {
+        if (charCount.has(char)) {
+            charCount.set(char, charCount.get(char) + 1);
+        } else {
+            charCount.set(char, 1);
+        }
+    }
+
+    for (const char of charCount.keys()) {
+        if (charCount.get(char) >= k) {
+            possibleChars.push(char);
+        }
+    }
+
+    // Test possibilities
+    let ans = "";
+    dfs("");
+
+    return ans;
+
+    // Recursive function, tests if the given subsequence repeats k times in s
+    function dfs(seq) {
+        // Does not have enough repeats, return
+        if (countRepeats(seq) < k) {
+            return;
+        }
+
+        // Update our answer if the new subsequence is better
+        if (seq.length > ans.length || (seq.length === ans.length && seq > ans)) {
+            ans = seq;
+        }
+
+        // Append possible characters to the subsequence and test again
+        if (seq.length < maxLen) {
+            for (const char of possibleChars) {
+                dfs(seq + char);
+            }
+        }
+    }
+
+    // Counts the number of times the given subsequence repeats in s (up to k)
+    function countRepeats(seq) {
+
+        // Empty string, return k
+        if (!seq) {
+            return k;
+        }
+
+        let repeats = 0;
+        let seqIdx = 0;
+
+        for (const char of s) {
+            if (char === seq[seqIdx]) {
+                seqIdx += 1;
+
+                if (seqIdx >= seq.length) {
+                    seqIdx = 0;
+                    repeats += 1;
+
+                    if (repeats >= k) {
+                        break;
+                    }
+                }
+            }
+        }
+
+        return repeats;
+    } 
+};