UDP (User Datagram Protocol) – Light wight but not that reliable


1. Packets size is smaller than TCP (udp header: 8 bytes vs tcp header 20bytes)

2. No connections to create and maintain

3. More control on when the data is sent


1. No error recovery. lost packets are just discarded.

2. Packets can arrive out of order

3. No traffic control.

4. Optional checksum in IPv4, only mandatory in IPv6

TCP (Transmission Control Protocol) – Reliable but bigger communication overhead

3 way handshake


1. Delivery acknowledgement. Receiver would send a respond confirming the packets has been received.

2. Retransmission. When the sender doesn’t get the confirmation from the receiver after a certain amount of time, it will resent the packets.

3. Traffic control. TCP delays transmission if the network is congested.

4. Checksum is now mandatory for both IPv6 and IPv4


1. Bigger header

2. Data doesn’t always get sent out immediately (think of video conferencing)

3. Bigger traffic overhead (because of the retransmission and delivery acknowledgement)


Dungeon Game

Dungeon Game

The demons had captured the princess (P) and imprisoned her in the bottom-right corner of a dungeon. The dungeon consists of M x N rooms laid out in a 2D grid. Our valiant knight (K) was initially positioned in the top-left room and must fight his way through the dungeon to rescue the princess.

The knight has an initial health point represented by a positive integer. If at any point his health point drops to 0 or below, he dies immediately.

Some of the rooms are guarded by demons, so the knight loses health (negative integers) upon entering these rooms; other rooms are either empty (0’s) or contain magic orbs that increase the knight’s health (positive integers).

In order to reach the princess as quickly as possible, the knight decides to move only rightward or downward in each step.

Write a function to determine the knight’s minimum initial health so that he is able to rescue the princess.

For example, given the dungeon below, the initial health of the knight must be at least 7 if he follows the optimal path RIGHT-> RIGHT -> DOWN -> DOWN.

-2 (K) -3 3
-5 -10 1
10 30 -5 (P)


  • The knight’s health has no upper bound.
  • Any room can contain threats or power-ups, even the first room the knight enters and the bottom-right room where the princess is imprisoned.
public int calculateMinimumHP(int[][] dungeon) {
        if (dungeon == null || dungeon.length == 0 || dungeon[0].length == 0) {
            return 0;
        int row = dungeon.length;
        int col = dungeon[0].length;
        //dp[i][j]: the minimum hp knight need to have at dungeon[i][j]
        int[][] dp = new int[row][col];
        //init bottom-right cell
        dp[row - 1][col - 1] = Math.max(1 - dungeon[row - 1][col - 1], 1);
        //init rightmost rooms
        for (int i = row - 2; i >= 0; i--) {
            dp[i][col - 1] =  Math.max(dp[i + 1][col - 1] - dungeon[i][col - 1], 1);
        //init bottom rooms
        for (int j = col - 2; j >= 0; j--) {
            dp[row - 1][j] = Math.max(dp[row - 1][j + 1] - dungeon[row - 1][j], 1);
        for (int i = row - 2; i >= 0; i--) {
            for (int j = col - 2; j >= 0; j--) {
                dp[i][j] = Math.max(Math.min(dp[i][j + 1] - dungeon[i][j], dp[i + 1][j] - dungeon[i][j]), 1);
        return dp[0][0];