Enclosed Islands - Google Top Interview Questions

Problem Statement :

You are given a two-dimensional integer matrix of 1s and 0s. 
A 1 represents land and 0 represents water. 
From any land cell you can move up, down, left or right to another land cell or go off the matrix.

Return the number of land cells from which we cannot go off the matrix.


n, m ≤ 250 where n and m are the number of rows and columns in matrix

Example 1


matrix = [
    [0, 0, 0, 1],
    [0, 1, 1, 0],
    [0, 1, 1, 0],
    [0, 0, 0, 0]




There's 4 land squares in the middle from which we cannot walk off the matrix.

Solution :


                        Solution in C++ :

def find_answer():
    for cell in matrix:
        if cell is edge_cell:
            push cell in queue

    while queue is not empty:
        current_cell = 0 # marking as visited
        for neighbor of current_cell:
            if neighbor is land:
                push neighbor in queue

    return  number of lands in matrix

                        Solution in Java :

import java.util.*;

class Solution {
    private int[][] matrix;
    public int solve(int[][] matrix) {
        this.matrix = matrix;
        // sink all islands touching an edge, then count land cells in interior.
        /// check for the vertical edges
        for (int i = 0; i < matrix.length; i++) {
            if (matrix[i][0] == 1) {
                floodfill(i, 0);
            if (matrix[i][matrix[0].length - 1] == 1) {
                floodfill(i, matrix[0].length - 1);
        // check for the horizontal edges
        for (int j = 0; j < matrix[0].length; j++) {
            if (matrix[0][j] == 1) {
                floodfill(0, j);
            if (matrix[matrix.length - 1][j] == 1) {
                floodfill(matrix.length - 1, j);
        int ret = 0;
        for (int i = 1; i < matrix.length - 1; i++) {
            for (int j = 1; j < matrix[0].length - 1; j++) {
                ret += matrix[i][j];
        return ret;
    public void floodfill(int i, int j) {
        if (i == -1 || j == -1 || i == matrix.length || j == matrix[0].length
            || matrix[i][j] == 0) {
        matrix[i][j] = 0;
        floodfill(i + 1, j);
        floodfill(i - 1, j);
        floodfill(i, j + 1);
        floodfill(i, j - 1);

                        Solution in Python : 
class Solution:
    def solve(self, matrix):
        q = [
            (i, j)
            for i in range(len(matrix))
            for j in range(len(matrix[i]))
            if matrix[i][j]
            and (i == 0 or i == len(matrix) - 1 or j == 0 or j == len(matrix[i]) - 1)
        idx = 0
        for x, y in q:
            matrix[x][y] = 0
        while idx < len(q):
            x, y = q[idx]
            for dx, dy in [(-1, 0), (0, -1), (0, 1), (1, 0)]:
                nx, ny = x + dx, y + dy
                if 0 <= nx < len(matrix) and 0 <= ny < len(matrix[nx]) and matrix[nx][ny]:
                    matrix[nx][ny] = 0
                    q.append((nx, ny))
            idx += 1
        return sum(sum(row) for row in matrix)

View More Similar Problems

Minimum Average Waiting Time

Tieu owns a pizza restaurant and he manages it in his own way. While in a normal restaurant, a customer is served by following the first-come, first-served rule, Tieu simply minimizes the average waiting time of his customers. So he gets to decide who is served first, regardless of how sooner or later a person comes. Different kinds of pizzas take different amounts of time to cook. Also, once h

View Solution →

Merging Communities

People connect with each other in a social network. A connection between Person I and Person J is represented as . When two persons belonging to different communities connect, the net effect is the merger of both communities which I and J belongs to. At the beginning, there are N people representing N communities. Suppose person 1 and 2 connected and later 2 and 3 connected, then ,1 , 2 and 3 w

View Solution →

Components in a graph

There are 2 * N nodes in an undirected graph, and a number of edges connecting some nodes. In each edge, the first value will be between 1 and N, inclusive. The second node will be between N + 1 and , 2 * N inclusive. Given a list of edges, determine the size of the smallest and largest connected components that have or more nodes. A node can have any number of connections. The highest node valu

View Solution →

Kundu and Tree

Kundu is true tree lover. Tree is a connected graph having N vertices and N-1 edges. Today when he got a tree, he colored each edge with one of either red(r) or black(b) color. He is interested in knowing how many triplets(a,b,c) of vertices are there , such that, there is atleast one edge having red color on all the three paths i.e. from vertex a to b, vertex b to c and vertex c to a . Note that

View Solution →

Super Maximum Cost Queries

Victoria has a tree, T , consisting of N nodes numbered from 1 to N. Each edge from node Ui to Vi in tree T has an integer weight, Wi. Let's define the cost, C, of a path from some node X to some other node Y as the maximum weight ( W ) for any edge in the unique path from node X to Y node . Victoria wants your help processing Q queries on tree T, where each query contains 2 integers, L and

View Solution →


We're going to make our own Contacts application! The application must perform two types of operations: 1 . add name, where name is a string denoting a contact name. This must store name as a new contact in the application. find partial, where partial is a string denoting a partial name to search the application for. It must count the number of contacts starting partial with and print the co

View Solution →