Coding

Concatenate One String to Another

Asked at

M2P Fintech

1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

The problem

You are given two strings str1 and str2. Write a program to concatenate str2 to the end of str1, and return the resulting combined string.

Concatenation means joining both strings in their original order.

Input: str1 = "Hello", str2 = "World" Output: "HelloWorld" Explanation: str2 is appended at the end of str1

**Input: **str1 = "abc", str2 = "123" **Output: **"abc123"

**Input: **str1 = "Good", str2 = "Morning"

1 <= str1.length, str2.length <= 105
Strings may contain alphabets, digits, spaces, or special characters

cpp

class Solution {
public:
    string concatenate(string str1, string str2) {
        // Your code goes here
    }
};

java

class Solution {
    public String concatenate(String str1, String str2) {
        // Your code goes here
    }
}

python

class Solution:
    def concatenate(self, str1: str, str2: str) -> str:
        # Your code goes here

javascript

class Solution {
    concatenate(str1, str2) {
        // Your code goes here
    }
}

csharp

class Solution {
    public string Concatenate(string str1, string str2) {
        // Your code goes here
    }
}

func concatenate(str1 string, str2 string) string {
	// Your code goes here
}

Stuck? Show a way to structure it+

01Preserve str1 order.
02Append str2 after it.
03Return the combined value.
04Use a builder where repeated concatenation is involved.

Reference answer

The required result is simply str1 + str2: copy the first string followed by the second. It takes O(|str1|+|str2|) time and output space because the result must contain both inputs. In languages with immutable strings, use a single concatenation or a string builder rather than repeatedly appending one character at a time. Do not insert a separator unless the specification asks for one. In an interview, make the invariant explicit before writing code, trace a boundary example that breaks a tempting shortcut, state what each stored value represents, and explain why every update preserves correctness. This verification step also surfaces empty-input, duplicate-value, and numeric-limit cases.

Then expect these follow-ups

How would you concatenate a large list of strings efficiently?
Tests: string builder
What is the difference between concatenation and interleaving?
Tests: problem reading

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