LeetCode 最小覆盖子串【滑动窗口】

叁歲伎倆 2022-09-10 12:08 187阅读 0赞

# 1 最小覆盖子串 #

给你一个字符串 s 、一个字符串 t 。返回 s 中涵盖 t 所有字符的最小子串。如果 s 中不存在涵盖 t 所有字符的子串，则返回空字符串 “” 。

**注意：**  
对于 t 中重复字符，我们寻找的子字符串中该字符数量必须不少于 t 中该字符数量。  
如果 s 中存在这样的子串，我们保证它是唯一的答案。

**示例 1**：

输入：s = "ADOBECODEBANC", t = "ABC"
    输出："BANC"

**示例 2**：

输入：s = "a", t = "a"
    输出："a"

**示例 3**:

输入: s = "a", t = "aa"
    输出: ""
    解释: t 中两个字符 'a' 均应包含在 s 的子串中，
    因此没有符合条件的子字符串，返回空字符串。

**提示**：

1 <= s.length, t.length <= 105
    s 和 t 由英文字母组成

# 2 解题思路 #

## 2.1 暴力 ##

**思路：** 找到大于目标长度所有子串，找出覆盖目标子串且长度最小的。

public String minWindow(String s, String t) { 
    
            String rs = "";
    
            //统计
            HashMap<Character, Integer> targetMap = new HashMap<>();
            for (int i = 0; i < t.length(); i++) { 
                char c = t.charAt(i);
                Integer count = targetMap.getOrDefault(c, 0);
                targetMap.put(c, count + 1);
            }
    
            //找子串
            for (int i = 0; i <= s.length() - t.length(); i++) { 
                for (int j = i + t.length() - 1; j <= s.length() - 1; j++) { 
                    Map<Character, Integer> tempMap = new HashMap<>();
                    for (int k = i; k <= j; k++) { 
                        char c = s.charAt(k);
                        Integer count = tempMap.getOrDefault(c, 0);
                        tempMap.put(c, count + 1);
                    }
                    //比较，选出最小的
                    if (checkCountMap(targetMap, tempMap) && (rs.length() > j - i || rs.equals(""))) { 
                        rs = s.substring(i, j+1);
                    }
                }
            }
    
            return rs;
        }
    
        private boolean checkCountMap(HashMap<Character, Integer> targetMap, Map<Character, Integer> tempMap) { 
            for (Map.Entry<Character, Integer> entry : targetMap.entrySet()) { 
                if (tempMap.getOrDefault(entry.getKey(), 0) < entry.getValue()) { 
                    return false;
                }
            }
            return true;
        }

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## 2.2 滑动窗口 ##

public String minWindow(String s, String t) { 
            String rs = "";
            //统计t中出现的次数
            Map<Character, Integer> targetMap = new HashMap<>();
            for (int i = 0; i < t.length(); i++) { 
                char c = t.charAt(i);
                Integer count = targetMap.getOrDefault(c, 0);
                targetMap.put(c, count + 1);
            }
            //滑动窗口
            int left = 0, right = t.length() - 1;
            while (left <= right && left <= s.length() - t.length() && right < s.length()) { 
                Map<Character, Integer> tempMap = new HashMap<>();
                for (int j = left; j <= right; j++) { 
                    char c = s.charAt(j);
                    Integer count = tempMap.getOrDefault(c, 0);
                    tempMap.put(c, count + 1);
                }
    
                if (checkCountMap(targetMap, tempMap)) { 
                    if (rs.equals("") || right - left + 1 < rs.length()) { 
                        rs = s.substring(left, right + 1);
                    }
    
                    //为了找出局部最小子串，left左移
                    left++;
                } else { 
                    right++;
                }
    
            }
            return rs;
        }
    
    
        private boolean checkCountMap(Map<Character, Integer> targetMap, Map<Character, Integer> tempMap) { 
            for (Map.Entry<Character, Integer> entry : targetMap.entrySet()) { 
                if (tempMap.getOrDefault(entry.getKey(), 0) < entry.getValue()) { 
                    return false;
                }
            }
            return true;
        }

![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5L2g5pC656eL5pyI5o-95pif5rKz5Li2_size_20_color_FFFFFF_t_70_g_se_x_16 1]

## 2.3 优化滑动窗口 ##

public String minWindow(String s, String t) { 
            String rs = "";
            //统计t中出现的次数
            Map<Character, Integer> targetMap = new HashMap<>();
            for (int i = 0; i < t.length(); i++) { 
                char c = t.charAt(i);
                Integer count = targetMap.getOrDefault(c, 0);
                targetMap.put(c, count + 1);
            }
            //滑动窗口
            int left = 0, right = 0;
            Map<Character, Integer> tempMap = new HashMap<>();
            while (left <= s.length() - t.length() && right < s.length()) { 
                char c = s.charAt(right);
                tempMap.put(c, tempMap.getOrDefault(c, 0) + 1);
                //判断
                while (checkCountMap(targetMap, tempMap)) { 
                    if (rs.equals("") || right - left + 1 < rs.length()) { 
                        rs = s.substring(left, right + 1);
                    }
                    //清除右指针
                    char temp = s.charAt(left);
                    tempMap.put(temp, tempMap.get(temp) - 1);
                    //左指针右移动
                    left++;
                }
                right++;
            }
            return rs;
        }
    
    
        private boolean checkCountMap(Map<Character, Integer> targetMap, Map<Character, Integer> tempMap) { 
            for (Map.Entry<Character, Integer> entry : targetMap.entrySet()) { 
                if (tempMap.getOrDefault(entry.getKey(), 0) < entry.getValue()) { 
                    return false;
                }
            }
            return true;
        }

![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5L2g5pC656eL5pyI5o-95pif5rKz5Li2_size_20_color_FFFFFF_t_70_g_se_x_16 2]

## 2.4 优化查询判断覆盖 ##

public String minWindow(String s, String t) { 
            String rs = "";
            //统计t中出现的次数
            Map<Character, Integer> targetMap = new HashMap<>();
            for (int i = 0; i < t.length(); i++) { 
                char c = t.charAt(i);
                Integer count = targetMap.getOrDefault(c, 0);
                targetMap.put(c, count + 1);
            }
            //滑动窗口
            int left = 0, right = 0;
            Map<Character, Integer> tempMap = new HashMap<>();
            int count = 0;
    
            while (left <= s.length() - t.length() && right < s.length()) { 
                char c = s.charAt(right);
    
                if (targetMap.containsKey(c)) { 
                    if (tempMap.getOrDefault(c, 0) < targetMap.get(c)) { 
                        count++;
                    }
                    tempMap.put(c, tempMap.getOrDefault(c, 0) + 1);
                }
    
                //判断
                while (count == t.length()) { 
                    if (rs.equals("") || right - left + 1 < rs.length()) { 
                        rs = s.substring(left, right + 1);
                    }
                    //清除右指针
                    char temp = s.charAt(left);
                    if (targetMap.containsKey(temp)) { 
                        tempMap.put(temp, tempMap.get(temp) - 1);
    
                        if (tempMap.getOrDefault(temp,0)<targetMap.get(temp)){ 
                            count--;
                        }
                    }
                    //左指针右移动
                    left++;
                }
                right++;
            }
            return rs;
        }

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