# Strings Interleave Problem

You are given three strings: a, b and i. Write a function that checks whether i is an interleaving of a and b.

String i is said to be interleaving string a and b, if:

len(i) = len(a) + len(b).

i only contains characters present in a or b.

i contains all characters of a. From a, any character a[index] should be added exactly once in i.

i contains all characters of b. From b, any character b[index] should be added exactly once in i.

Order of all characters in individual strings (a and b) is preserved.

Example One

Input:

a = "123"

b = "456"

i = "123456"

Output: True

Example Two

Input:

a = "AAB"

b = "AAC"

i = "AAAABC"

Output: True

Example Three

Input:

a = "AAB"

b = "AAC"

i = "AAABAC"

Output: True

Notes

Input Parameters: You will be given three strings a, b and i.

Strings can contain:

Small alphabets - a-z

Large alphabets - A-Z

Numbers - 0-9

Output: Return a boolean if i is an interleaving of a and b.

Constraints:

● 0

● 0

#### Solution

Brute force recursive solution

First character in i should match first character in a or first character in b.

* If it matches first character in a, try matching second character in i with second character in a or first character in b

* If it matches first character in b, try matching second character in i with second character in b or first character in a

* If it matches none of them, terminate with a false

Hence, keep recursing for the rest of the strings. This is an exponential solution, O(2^(len(a)+len(b))) as you can either pick a character from a or a character from b.

#### Optimal Solution

Convention: str[0 : x] = first x chars of str.

We can say that i[0 : (a_i + b_i)] is an interleaving of a[0 : a_i] and b[0 : b_i], if at least one of the below two is true:

1) - i[0 : (a_i + b_i - 1)] should be an interleaving of a[0 : (a_i - 1)] and b[0 : b_i]

and

- a[ai - 1] == i[ai + bi - 1].

or

2) - i[0 : (a_i + b_i - 1)] should be an interleaving of a[0 : a_i] and b[0 : (b_i - 1)]

and

- b[bi - 1] == i[ai + bi - 1].

We can use DP to keep track of the already calculated values. Have a look at the solution for more details.

Space Complexity:

O(len(a) * len(b))

Time Complexity:

O(len(a) * len(b))

``````
// -------- START --------

bool doStringsInterleave(string a, string b, string i) {
if (a.length() + b.length() != i.length()){
return false;
}

/*
Convention: str[0 : x] = first x chars of str.

dp[a_i][b_i] =  True if i[0 : (a_i + b_i)] is an interleaving
of a[0 : a_i] and b[0 : b_i], else False.
*/
bool dp[a.length() + 1][b.length() + 1];

for (int ai = 0; ai <= a.length(); ai++){
for (int bi = 0; bi <= b.length(); bi++){
/*
To dp[a_i][b_i] be true, at least one of the below two should be true:

1) i[0 : (a_i + b_i - 1)] should be an interleaving
of a[0 : (a_i - 1)] and b[0 : b_i]
and a[ai - 1] == i[ai + bi - 1].

2) i[0 : (a_i + b_i - 1)] should be an interleaving
of a[0 : a_i] and b[0 : (b_i - 1)]
and b[bi - 1] == i[ai + bi - 1].
*/

// Keeps track of the above mentioned #1
bool dp_ai_min_1_bi = false;
// Keeps track of the above mentioned #2
bool dp_ai_bi_min_1 = false;

if (ai > 0){
dp_ai_min_1_bi = dp[ai - 1][bi] &&
(a[ai - 1] == i[ai + bi - 1]);
}
if (bi > 0){
dp_ai_bi_min_1 = dp[ai][bi - 1] &&
(b[bi - 1] == i[ai + bi - 1]);
}

// dp = Will be true because empty string is an interleaving
// of two empty strings.
dp[ai][bi] = (ai == 0 && bi == 0) ||
dp_ai_min_1_bi ||
dp_ai_bi_min_1;
}
}

return dp[a.length()][b.length()];
}

// -------- END --------
``````

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