feat: Partition Labels

Author: hongbo-miao

JavaScript/0763. Partition Labels.js

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+// A string S of lowercase letters is given. We want to partition this string into as many parts as possible so that each letter appears in at most one part, and return a list of integers representing the size of these parts.
+//
+// Example 1:
+// Input: S = "ababcbacadefegdehijhklij"
+// Output: [9,7,8]
+// Explanation:
+// The partition is "ababcbaca", "defegde", "hijhklij".
+// This is a partition so that each letter appears in at most one part.
+// A partition like "ababcbacadefegde", "hijhklij" is incorrect, because it splits S into less parts.
+//
+// Note:
+//
+// S will have length in range [1, 500].
+// S will consist of lowercase letters ('a' to 'z') only.
+
+/**
+ * @param {string} S
+ * @return {number[]}
+ */
+
+/** Greedy */
+// Let's try to repeatedly choose the smallest left-justified partition.
+// Consider the first label, say it's 'a'. The first partition must include it, and also the last occurrence of 'a'.
+// However, between those two occurrences of 'a', there could be other labels that make the minimum size of this partition bigger.
+// For example, in "abccaddbeffe", the minimum first partition is "abccaddb".
+// This gives us the idea for the algorithm: For each letter encountered, process the last occurrence of that letter,
+// extending the current partition [start, end] appropriately.
+function partitionLabels(S) {
+  if (S == null || S.length === 0) return null;
+
+  const res = [];
+  const map = {};  // record the last seen index of the each char
+
+  for (let i = 0; i < S.length; i++) {
+    map[S[i]] = i;
+  }
+
+  // record the end index of the current sub string
+  let last = 0;
+  let start = 0;
+  for (let i = 0; i < S.length; i++) {
+    last = Math.max(last, map[S[i]]);
+    if (last === i) {
+      res.push(last - start + 1);
+      start = last + 1;
+    }
+  }
+  return res;
+}