update 646, cheatsheet

Author: yennanliu

README.md

@@ -942,7 +942,7 @@
 452 | [Minimum Number of Arrows to Burst Balloons](https://leetcode.com/problems/minimum-number-of-arrows-to-burst-balloons/) | [Python](./leetcode_python/Greedy/minimum-number-of-arrows-to-burst-balloons.py) | _O(nlogn)_ | _O(1)_ | Medium | | AGAIN* 
 455 | [Assign Cookies](https://leetcode.com/problems/assign-cookies/) | [Python](./leetcode_python/Greedy/assign-cookies.py) | _O(nlogn)_ | _O(1)_ | Easy | | OK* 
 621 | [Task Scheduler](https://leetcode.com/problems/task-scheduler/) | [Python](./leetcode_python/Greedy/task-scheduler.py)  | _O(n)_ | _O(1)_ | Medium |`trick`,`array`,`greedy`, `fb` | AGAIN****** (6)
-646 | [Maximum Length of Pair Chain](https://leetcode.com/problems/maximum-length-of-pair-chain/) | [Python](./leetcode_python/Greedy/maximum-length-of-pair-chain.py)  | _O(nlogn)_ | _O(1)_ | Medium | good basic, similar as `#435 Non-overlapping Intervals`, Line Sweep, `amazon` |  AGAIN*** (3)
+646 | [Maximum Length of Pair Chain](https://leetcode.com/problems/maximum-length-of-pair-chain/) | [Python](./leetcode_python/Greedy/maximum-length-of-pair-chain.py)  | _O(nlogn)_ | _O(1)_ | Medium | good trick, greedy, similar as `#435 Non-overlapping Intervals`, Line Sweep, `amazon` |  OK****** (4) (but again)
 649 | [Dota2 Senate](https://leetcode.com/problems/dota2-senate/) | [Python](./leetcode_python/Greedy/dota2-senate.py)  | _O(n)_ | _O(n)_ | Medium | complex question | AGAIN (not start*)
 659 | [Split Array into Consecutive Subsequences](https://leetcode.com/problems/split-array-into-consecutive-subsequences/) | [Python](./leetcode_python/Greedy/split-array-into-consecutive-subsequences.py)  | _O(n)_ | _O(1)_ | Medium | | AGAIN (not start*)
 738 | [Monotone Increasing Digits](https://leetcode.com/problems/monotone-increasing-digits/) | [Python](./leetcode_python/Greedy/monotone-increasing-digits.py) | _O(1)_ | _O(1)_ | Medium | good trick, greedy, string, trick, `Amazon`| AGAIN********* (4)

doc/cheatsheet/greedy.md

@@ -152,4 +152,50 @@ class Solution(object):
             # Be aware of it : if there is no valid "v", then the while loop will break automatically at this condition (stop = True)
             if stop: break
         return ans[1:] if len(ans[1:]) == len(S) else ''
+```
+
+### 2-7) Maximum Length of Pair Chain
+```python
+# LC 646 Maximum Length of Pair Chain
+# V0 
+# IDEA : GREEDY + sorting
+# ->  we sort on pair's 1st element -> possible cases that we can get sub pairs with max length with the needed conditions
+# ->  we need to find the "max length" of "continous or non-continous" sub pairs (with condition)
+#      -> so start from the "sorted 1st pair" CAN ALWAYS MAKE US GET THE MAX LENGTH of sub pairs with the condition ( we define a pair (c, d) can follow another pair (a, b) if and only if b < c. Chain of pairs can be formed in this fashion.)
+##########
+# DEMO
+#     ...: 
+#     ...: x = [[-10,-8],[8,9],[-5,0],[6,10],[-6,-4],[1,7],[9,10],[-4,7]]
+#     ...: s = Solution()
+#     ...: r = s.findLongestChain(x)
+#     ...: print (r)
+# pairs = [[-10, -8], [-6, -4], [-5, 0], [1, 7], [-4, 7], [8, 9], [6, 10], [9, 10]]
+# x = -10 y = -8
+#  currTime = -8 ans = 1
+# x = -6 y = -4
+#  currTime = -4 ans = 2
+# x = -5 y = 0
+#  currTime = -4 ans = 2
+# x = 1 y = 7
+#  currTime = 7 ans = 3
+# x = -4 y = 7
+#  currTime = 7 ans = 3
+# x = 8 y = 9
+#  currTime = 9 ans = 4
+# x = 6 y = 10
+#  currTime = 9 ans = 4
+# x = 9 y = 10
+#  currTime = 9 ans = 4
+# 4
+class Solution(object):
+    def findLongestChain(self, pairs):
+        pairs = sorted(pairs, key=lambda x : x[1])
+        ### NOTICE HERE
+        currTime, ans = float('-inf'), 0
+        for x, y in pairs:
+            ### NOTICE HERE
+            if currTime < x:
+                currTime = y
+                ans += 1
+        return ans
 ```

doc/progress.md

@@ -9,7 +9,8 @@
 			- (normal order)
 			- (pdf : p.13)
 				- 776 (again!)
-				- 646
+				- 646 (again!)
+				- 435
 			- =======
 			- **back track**
 			- Binary Search Tree

leetcode_python/Greedy/maximum-length-of-pair-chain.py

@@ -1,5 +1,68 @@
+"""
+
+646. Maximum Length of Pair Chain
+Medium
+
+You are given an array of n pairs pairs where pairs[i] = [lefti, righti] and lefti < righti.
+
+A pair p2 = [c, d] follows a pair p1 = [a, b] if b < c. A chain of pairs can be formed in this fashion.
+
+Return the length longest chain which can be formed.
+
+You do not need to use up all the given intervals. You can select pairs in any order.
+
+ 
+
+Example 1:
+
+Input: pairs = [[1,2],[2,3],[3,4]]
+Output: 2
+Explanation: The longest chain is [1,2] -> [3,4].
+Example 2:
+
+Input: pairs = [[1,2],[7,8],[4,5]]
+Output: 3
+Explanation: The longest chain is [1,2] -> [4,5] -> [7,8].
+ 
+
+Constraints:
+
+n == pairs.length
+1 <= n <= 1000
+-1000 <= lefti < righti <= 1000
+
+"""
+
 # V0 
-# IDEA : GREEDY
+# IDEA : GREEDY + sorting
+# ->  we sort on pair's 1st element -> possible cases that we can get sub pairs with max length with the needed conditions
+# ->  we need to find the "max length" of "continous or non-continous" sub pairs (with condition)
+#      -> so start from the "sorted 1st pair" CAN ALWAYS MAKE US GET THE MAX LENGTH of sub pairs with the condition ( we define a pair (c, d) can follow another pair (a, b) if and only if b < c. Chain of pairs can be formed in this fashion.)
+##########
+# DEMO
+#     ...: 
+#     ...: x = [[-10,-8],[8,9],[-5,0],[6,10],[-6,-4],[1,7],[9,10],[-4,7]]
+#     ...: s = Solution()
+#     ...: r = s.findLongestChain(x)
+#     ...: print (r)
+# pairs = [[-10, -8], [-6, -4], [-5, 0], [1, 7], [-4, 7], [8, 9], [6, 10], [9, 10]]
+# x = -10 y = -8
+#  currTime = -8 ans = 1
+# x = -6 y = -4
+#  currTime = -4 ans = 2
+# x = -5 y = 0
+#  currTime = -4 ans = 2
+# x = 1 y = 7
+#  currTime = 7 ans = 3
+# x = -4 y = 7
+#  currTime = 7 ans = 3
+# x = 8 y = 9
+#  currTime = 9 ans = 4
+# x = 6 y = 10
+#  currTime = 9 ans = 4
+# x = 9 y = 10
+#  currTime = 9 ans = 4
+# 4
 class Solution(object):
     def findLongestChain(self, pairs):
         pairs = sorted(pairs, key=lambda x : x[1])