[Function add]

1. Add leetcode solutions with tag amazon.

Author: Seanforfun

leetcode/323. Number of Connected Components in an Undirected Graph.md

@@ -58,3 +58,35 @@ You can assume that no duplicate edges will appear in edges. Since all edges are
 		}
 	}
 	```
+
+### Amazon session
+* Method 1: union find
+	```Java
+	class Solution {
+		private int[] uf;
+		public int countComponents(int n, int[][] edges) {
+			this.uf = new int[n];
+			for(int i = 0; i < n; i++)
+				uf[i] = i;
+			for(int[] edge : edges){
+				union(edge[0], edge[1]);
+			}
+			int count = 0;
+			for(int i = 0; i < n; i++)
+				if(uf[i] == i)
+					count++;
+			return count;
+		}
+		private int find(int i){
+			if(uf[i] != i){
+				uf[i] = find(uf[i]);
+			}
+			return uf[i];
+		}
+		private void union(int i, int j){
+			int x = find(i);
+			int y = find(j);
+			uf[x] = y;
+		}
+	}
+	```

leetcode/983. Minimum Cost For Tickets.md

@@ -0,0 +1,69 @@
+## 983. Minimum Cost For Tickets
+
+### Question
+In a country popular for train travel, you have planned some train travelling one year in advance.  The days of the year that you will travel is given as an array days.  Each day is an integer from 1 to 365.
+
+Train tickets are sold in 3 different ways:
+* a 1-day pass is sold for costs[0] dollars;
+* a 7-day pass is sold for costs[1] dollars;
+* a 30-day pass is sold for costs[2] dollars.
+
+The passes allow that many days of consecutive travel.  For example, if we get a 7-day pass on day 2, then we can travel for 7 days: day 2, 3, 4, 5, 6, 7, and 8.
+
+Return the minimum number of dollars you need to travel every day in the given list of days.
+
+```
+Example 1:
+
+Input: days = [1,4,6,7,8,20], costs = [2,7,15]
+Output: 11
+Explanation: 
+For example, here is one way to buy passes that lets you travel your travel plan:
+On day 1, you bought a 1-day pass for costs[0] = $2, which covered day 1.
+On day 3, you bought a 7-day pass for costs[1] = $7, which covered days 3, 4, ..., 9.
+On day 20, you bought a 1-day pass for costs[0] = $2, which covered day 20.
+In total you spent $11 and covered all the days of your travel.
+
+Example 2:
+
+Input: days = [1,2,3,4,5,6,7,8,9,10,30,31], costs = [2,7,15]
+Output: 17
+Explanation: 
+For example, here is one way to buy passes that lets you travel your travel plan:
+On day 1, you bought a 30-day pass for costs[2] = $15 which covered days 1, 2, ..., 30.
+On day 31, you bought a 1-day pass for costs[0] = $2 which covered day 31.
+In total you spent $17 and covered all the days of your travel.
+```
+ 
+
+Note:
+1. 1 <= days.length <= 365
+2. 1 <= days[i] <= 365
+3. days is in strictly increasing order.
+4. costs.length == 3
+5. 1 <= costs[i] <= 1000
+
+### Solutions
+* Method 1: dp
+	1. transfer function:
+	2. for every single day, the we check if it deserves to by a long term ticket.
+	```Java
+	class Solution {
+		public int mincostTickets(int[] days, int[] costs) {
+			int last = days[days.length - 1];
+			int[] dp = new int[last + 1];
+			int index = 0;
+			for(int i = 1; i <= last; i++){
+				if(i != days[index]){
+					dp[i] = dp[i - 1];
+					continue;
+				}
+				dp[i] = dp[i - 1] + costs[0]; // buy 1 day pass.
+				dp[i] = Math.min(dp[i], (i - 7 >= 0 ? dp[i - 7]: 0) + costs[1]);    // buy 7 day pass.
+				dp[i] = Math.min(dp[i], (i - 30 >= 0 ? dp[i - 30]: 0) + costs[2]);  // buy 30 day pass.
+				index++;
+			}
+			return dp[last];
+		}
+	}
+	```