030-substring-with-concatenation-of-all-words

030-substring-with-concatenation-of-all-words.playground/Contents.swift

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+/// 30. Substring with Concatenation of All Words
+/// You are given a string, s, and a list of words, words, that are all of the same length. Find all
+/// starting indices of substring(s) in s that is a concatenation of each word in words exactly once
+/// and without any intervening characters.
+///
+/// Example:
+/// Input:
+/// s = "barfoothefoobarman",
+/// words = ["foo","bar"]
+/// Output: [0,9]
+/// Explanation: Substrings starting at index 0 and 9 are "barfoor" and "foobar" respectively.
+///     The output order does not matter, returning [9,0] is fine too.
+
+import XCTest
+
+// Approach: Two Pointers
+func findSubstring(_ s: String, _ words: [String]) -> [Int] {
+    guard !words.isEmpty else { return [] }
+    let wordSize = words[0].count
+    let windowSize = wordSize * words.count
+    guard s.count >= windowSize else { return [] }
+
+    // Build a counted set for words
+    var wordCounts: [String: Int] = [:]
+    words.forEach { wordCounts[$0, default: 0] += 1}
+
+    let chars = Array(s) // substring is extremly slow, so use character array here
+    var result: [Int] = []
+    for i in 0..<wordSize {
+        var left = i
+        var right = i
+        var counts: [String: Int] = wordCounts
+        var found = 0
+        while right <= chars.count - wordSize {
+            let word = String(chars[right..<right + wordSize])
+            right += wordSize
+            if let count = counts[word] {
+                counts[word] = count - 1
+                if count == 1 {
+                    found += 1
+                }
+            }
+
+            if found == counts.count {
+                result.append(left)
+            }
+
+            guard right - left == windowSize else { continue }
+            let head = String(chars[left..<left + wordSize])
+            if let count = counts[head] {
+                counts[head] = count + 1
+                if count == 0 {
+                    found -= 1
+                }
+            }
+            left += wordSize
+        }
+    }
+    return result
+}
+
+class Tests: XCTestCase {
+    func testExample1() {
+        let s = "barfoothefoobarman"
+        let words = ["foo","bar"]
+        XCTAssertEqual(findSubstring(s, words), [0, 9])
+    }
+
+    func testExample2() {
+        let s = "wordgoodgoodgoodbestword"
+        let words = ["word", "good", "best", "good"]
+        XCTAssertEqual(findSubstring(s, words), [8])
+    }
+}
+
+Tests.defaultTestSuite.run()

030-substring-with-concatenation-of-all-words.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='macos' executeOnSourceChanges='false'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

README.md

@@ -45,3 +45,4 @@
 27 | [Remove Element](https://leetcode.com/problems/remove-element/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/027-remove-element.playground/Contents.swift) | Two Pointers
 28 | [Implement strStr()](https://leetcode.com/problems/implement-strstr/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/028-implement-strstr.playground/Contents.swift) | String
 29 | [Divide Two Integers](https://leetcode.com/problems/divide-two-integers/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/029-divide-two-integers.playground/Contents.swift) | String
+29 | [Substring with Concatenation of All Words](https://leetcode.com/problems/substring-with-concatenation-of-all-words/description/) | [Solution](https://github.com/zhubofei/LeetCode-Swift/blob/master/030-substring-with-concatenation-of-all-words.playground/Contents.swift) | Two Pointers