【LeetCode】76. Minimum Window Substring 解題報告

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76. Minimum Window Substring / Hard

Given two strings s and t of lengths m and n respectively, 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 “”.

The testcases will be generated such that the answer is unique.

A substring is a contiguous sequence of characters within the string.

Example 1:

Input: s = “ADOBECODEBANC”, t = “ABC”
Output: “BANC”
Explanation: The minimum window substring “BANC” includes ‘A’, ‘B’, and ‘C’ from string t.

Example 2:

Input: s = “a”, t = “a”
Output: “a”
Explanation: The entire string s is the minimum window.

Example 3:

Input: s = “a”, t = “aa”
Output: “”
Explanation: Both ‘a’s from t must be included in the window.
Since the largest window of s only has one ‘a’, return empty string.

Constraints:

  • m == s.length
  • n == t.length
  • 1 <= m, n <= 10^5
  • s and t consist of uppercase and lowercase English letters.

Solution: Map + Sliding Window

思路

先利用 Hashmap 將字串 t 的字元出現次數都存下來，
接下來不斷移動右邊的指標，用 cnt 來記錄扣對（s 的字元也出現在 t 而且沒有扣超過的狀況）的情況，
也就是

–hmap[s[r]] >= 0
如果 cnt 和 t 的長度相等，代表目前的範圍已經涵蓋了 s 所有的字元。
這時，讓左指標往右走，計算長度是否是目前最短的，同時將剛剛扣掉的字元從 hashmap 加回來。

效能

Complexity

  • Time Complexity: O(M+N), which M and N are the length of string S and T
  • Space Complexity: O(M+N)

LeetCode Result

Code

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class Solution {
public:
    string minWindow(string s, string t) {
        int hmap[128] = {0};
        int l = 0, r = 0, cnt = 0;
        int min_len = INT_MAX, min_start = -1;

        for(const auto& c: t) ++hmap[c];
        for(int r = 0; r < s.size(); ++r) {
            if(--hmap[s[r]] >= 0) ++cnt;
            while(cnt == t.size()) {
                if(r-l+1 < min_len) {
                    min_start = l;
                    min_len = r-l+1;
                }
                if(++hmap[s[l]] > 0) --cnt;
                ++l;
            }
        }
        return min_start == -1 ? "" : s.substr(min_start, min_len);
    }
};