Shortest Substring Containing Characters

hardcodingAlgorithmsSliding WindowStringsHash Map~40 min

Asked at Google, Meta, Stripe

Overview

Given two strings s and t, return the minimum window substring of s such that every character in t (including duplicates) is included in the window. If there is no such substring, return the empty string "".

Constraints

1 <= s.length, t.length <= 100,000
s and t consist of uppercase and lowercase English letters.
Must run in O(n) time.

Examples

minWindow("ADOBECODEBANC", "ABC");
// => "BANC"

minWindow("a", "a");
// => "a"

minWindow("a", "aa");
// => ""  (not enough 'a's)

Solution

Reveal solution

function minWindow(s, t) {
  const need = {};
  for (const c of t) need[c] = (need[c] || 0) + 1;

  let have = 0, required = Object.keys(need).length;
  const window = {};
  let left = 0, minLen = Infinity, minStart = 0;

  for (let right = 0; right < s.length; right++) {
    const c = s[right];
    window[c] = (window[c] || 0) + 1;

    if (need[c] && window[c] === need[c]) have++;

    while (have === required) {
      if (right - left + 1 < minLen) {
        minLen = right - left + 1;
        minStart = left;
      }
      const lc = s[left];
      window[lc]--;
      if (need[lc] && window[lc] < need[lc]) have--;
      left++;
    }
  }

  return minLen === Infinity ? "" : s.substring(minStart, minStart + minLen);
}

Sliding window: expand right to include all required characters, then shrink left to find the minimum window. Track how many unique characters are satisfied.

Resources

shortest-substring-containing-characters.js

Shortest Substring Containing Characters

hardcodingAlgorithmsSliding Window

Overview

Given two strings s and t, return the minimum window substring of s such that every character in t (including duplicates) is included in the window. If there is no such substring, return the empty string "".

Constraints

1 <= s.length, t.length <= 100,000
s and t consist of uppercase and lowercase English letters.
Must run in O(n) time.

Examples

minWindow("ADOBECODEBANC", "ABC");
// => "BANC"

minWindow("a", "a");
// => "a"

minWindow("a", "aa");
// => ""  (not enough 'a's)

Solution

Reveal solution

function minWindow(s, t) {
  const need = {};
  for (const c of t) need[c] = (need[c] || 0) + 1;

  let have = 0, required = Object.keys(need).length;
  const window = {};
  let left = 0, minLen = Infinity, minStart = 0;

  for (let right = 0; right < s.length; right++) {
    const c = s[right];
    window[c] = (window[c] || 0) + 1;

    if (need[c] && window[c] === need[c]) have++;

    while (have === required) {
      if (right - left + 1 < minLen) {
        minLen = right - left + 1;
        minStart = left;
      }
      const lc = s[left];
      window[lc]--;
      if (need[lc] && window[lc] < need[lc]) have--;
      left++;
    }
  }

  return minLen === Infinity ? "" : s.substring(minStart, minStart + minLen);
}

Sliding window: expand right to include all required characters, then shrink left to find the minimum window. Track how many unique characters are satisfied.

Resources

NameTopicDifficulty

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