Solutions

Prerequisites

Before attempting this problem, you should be comfortable with:

String Matching - Finding all occurrences of substrings within a larger string
Boolean Arrays for Marking - Using a boolean array to mark positions that satisfy certain conditions
Interval Merging Concepts - Understanding how overlapping or adjacent marked regions should be merged together

1. Mark Bold Characters

Intuition

The key insight is that we need to track which characters should be bold, not which substrings. If we find all occurrences of each word in the string and mark every character position that falls within any match, we can then merge overlapping or adjacent bold regions naturally.

We use a boolean array where each index corresponds to a character in the string. For every word, we find all its occurrences and mark the corresponding positions as bold. When building the result, we only insert  at the start of a bold region and  at the end, which handles merging automatically.

Algorithm

Create a boolean array bold of size n (length of string s), initialized to false.
For each word in the dictionary:
- Find all occurrences of the word in s.
- For each occurrence starting at index start, mark bold[i] = true for all i from start to start + len(word) - 1.
Build the result string by iterating through s:
- If bold[i] is true and either i == 0 or bold[i-1] is false, insert .
- Append the character s[i].
- If bold[i] is true and either i == n-1 or bold[i+1] is false, insert .
Return the result string.

class Solution:
    def addBoldTag(self, s: str, words: List[str]) -> str:
        n = len(s)
        bold = [False] * n
        
        for word in words:
            start = s.find(word)
            while start != -1:
                for i in range(start, start + len(word)):
                    bold[i] = True
                    
                start = s.find(word, start + 1)

        open_tag = "<b>"
        close_tag = "</b>"
        ans = []
        
        for i in range(n):
            if bold[i] and (i == 0 or not bold[i - 1]):
                ans.append(open_tag)
                
            ans.append(s[i])
            
            if bold[i] and (i == n - 1 or not bold[i + 1]):
                ans.append(close_tag)
        
        return "".join(ans)

class Solution {
    public String addBoldTag(String s, String[] words) {
        int n = s.length();
        boolean[] bold = new boolean[n];
        
        for (String word: words) {
            int start = s.indexOf(word);
            while (start != -1) {
                for (int i = start; i < start + word.length(); i++) {
                    bold[i] = true;
                }
                
                start = s.indexOf(word, start + 1);
            }
        }
        
        String openTag = "<b>";
        String closeTag = "</b>";
        StringBuilder ans = new StringBuilder();
        
        for (int i = 0; i < n; i++) {
            if (bold[i] && (i == 0 || !bold[i - 1])) {
                ans.append(openTag);
            }
            
            ans.append(s.charAt(i));
            
            if (bold[i] && (i == n - 1 || !bold[i + 1])) {
                ans.append(closeTag);
            }
        }
        
        return ans.toString();
    }
}

class Solution {
public:
    string addBoldTag(string s, vector<string>& words) {
        int n = s.size();
        vector<bool> bold(n);
        
        for (string word: words) {
            int start = s.find(word);
            while (start != -1) {
                for (int i = start; i < start + word.size(); i++) {
                    bold[i] = true;
                }
                
                start = s.find(word, start + 1);
            }
        }
        
        string openTag = "<b>";
        string closeTag = "</b>";
        string ans = "";
        
        for (int i = 0; i < n; i++) {
            if (bold[i] && (i == 0 || !bold[i - 1])) {
                ans += openTag;
            }
            
            ans += s[i];
            
            if (bold[i] && (i == n - 1 || !bold[i + 1])) {
                ans += closeTag;
            }
        }
        
        return ans;
    }
};

class Solution {
    /**
     * @param {string} s
     * @param {string[]} words
     * @return {string}
     */
    addBoldTag(s, words) {
        const n = s.length;
        const bold = new Array(n).fill(false);

        for (const word of words) {
            let start = s.indexOf(word);
            while (start !== -1) {
                for (let i = start; i < start + word.length; i++) {
                    bold[i] = true;
                }
                start = s.indexOf(word, start + 1);
            }
        }

        const openTag = "<b>";
        const closeTag = "</b>";
        const ans = [];

        for (let i = 0; i < n; i++) {
            if (bold[i] && (i === 0 || !bold[i - 1])) {
                ans.push(openTag);
            }
            ans.push(s[i]);
            if (bold[i] && (i === n - 1 || !bold[i + 1])) {
                ans.push(closeTag);
            }
        }

        return ans.join("");
    }
}

public class Solution {
    public string AddBoldTag(string s, string[] words) {
        int n = s.Length;
        bool[] bold = new bool[n];

        foreach (string word in words) {
            int start = s.IndexOf(word);
            while (start != -1) {
                for (int i = start; i < start + word.Length; i++) {
                    bold[i] = true;
                }
                start = s.IndexOf(word, start + 1);
            }
        }

        string openTag = "<b>";
        string closeTag = "</b>";
        StringBuilder ans = new StringBuilder();

        for (int i = 0; i < n; i++) {
            if (bold[i] && (i == 0 || !bold[i - 1])) {
                ans.Append(openTag);
            }
            ans.Append(s[i]);
            if (bold[i] && (i == n - 1 || !bold[i + 1])) {
                ans.Append(closeTag);
            }
        }

        return ans.ToString();
    }
}

func addBoldTag(s string, words []string) string {
    n := len(s)
    bold := make([]bool, n)

    for _, word := range words {
        start := 0
        for {
            idx := strings.Index(s[start:], word)
            if idx == -1 {
                break
            }
            idx += start
            for i := idx; i < idx+len(word); i++ {
                bold[i] = true
            }
            start = idx + 1
        }
    }

    openTag := "<b>"
    closeTag := "</b>"
    var ans strings.Builder

    for i := 0; i < n; i++ {
        if bold[i] && (i == 0 || !bold[i-1]) {
            ans.WriteString(openTag)
        }
        ans.WriteByte(s[i])
        if bold[i] && (i == n-1 || !bold[i+1]) {
            ans.WriteString(closeTag)
        }
    }

    return ans.String()
}

class Solution {
    fun addBoldTag(s: String, words: Array<String>): String {
        val n = s.length
        val bold = BooleanArray(n)

        for (word in words) {
            var start = s.indexOf(word)
            while (start != -1) {
                for (i in start until start + word.length) {
                    bold[i] = true
                }
                start = s.indexOf(word, start + 1)
            }
        }

        val openTag = "<b>"
        val closeTag = "</b>"
        val ans = StringBuilder()

        for (i in 0 until n) {
            if (bold[i] && (i == 0 || !bold[i - 1])) {
                ans.append(openTag)
            }
            ans.append(s[i])
            if (bold[i] && (i == n - 1 || !bold[i + 1])) {
                ans.append(closeTag)
            }
        }

        return ans.toString()
    }
}

class Solution {
    func addBoldTag(_ s: String, _ words: [String]) -> String {
        let chars = Array(s)
        let n = chars.count
        var bold = [Bool](repeating: false, count: n)

        for word in words {
            var searchStart = s.startIndex
            while let range = s.range(of: word, range: searchStart..<s.endIndex) {
                let start = s.distance(from: s.startIndex, to: range.lowerBound)
                let end = s.distance(from: s.startIndex, to: range.upperBound)
                for i in start..<end {
                    bold[i] = true
                }
                searchStart = s.index(after: range.lowerBound)
            }
        }

        let openTag = "<b>"
        let closeTag = "</b>"
        var ans = ""

        for i in 0..<n {
            if bold[i] && (i == 0 || !bold[i - 1]) {
                ans += openTag
            }
            ans.append(chars[i])
            if bold[i] && (i == n - 1 || !bold[i + 1]) {
                ans += closeTag
            }
        }

        return ans
    }
}

Time & Space Complexity

The time complexity may differ between languages. It is dependent on how the built-in method is implemented.
For this analysis, we will assume that we are using Java.

Time complexity: $O(m \cdot (n^2 \cdot k - n \cdot k^2))$
Space complexity: $O(n)$

Where $n$ is s.length, $m$ is words.length, and $k$ is the average length of the words.

Common Pitfalls

Not Merging Adjacent Bold Regions

A common mistake is inserting separate  tags for each word match without merging overlapping or adjacent regions. For example, if "ab" and "bc" both match in "abc", the result should be abc, not abbc.

# Wrong: treating each match independently
for word in words:
    s = s.replace(word, f"<b>{word}</b>")  # Creates nested/separate tags

Off-by-One When Marking Bold Ranges

When marking character positions as bold, forgetting that the end index should be start + len(word) - 1 (inclusive) or using start + len(word) (exclusive) inconsistently leads to incorrect bold boundaries.

# Wrong: marking one character too many or too few
for i in range(start, start + len(word) + 1):  # Off-by-one error
    bold[i] = True

Missing Overlapping Occurrences of the Same Word

Using find() with start + len(word) as the next search position misses overlapping matches. For example, searching for "aa" in "aaa" should find two matches (at index 0 and 1), but skipping by word length finds only one.

# Wrong: skipping by word length misses overlaps
start = s.find(word, start + len(word))  # Should be start + 1