259_3Sum_Smaller

Author: qiyuangong

README.md

@@ -75,5 +75,6 @@
 | 228 | [Summary Ranges](https://leetcode.com/problems/summary-ranges/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/228_Summary_Ranges.py) | Detect start and jump, O(n) and O(1) |
 | 243 | [Shortest Word Distance](https://leetcode.com/problems/shortest-word-distance/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/243_Shortest_Word_Distance.py) | Update index1 and index2, and check distance, O(n) and O(1) |
 | 252 | [Meeting Rooms](https://leetcode.com/problems/meeting-rooms/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/252_Meeting_Rooms.py) | 1. Sort and compare intervals[i].end with intervals[i+1], O(nlogn) and O(1)<br>2. Sort and check intervals when count >= 2, O(nlogn) and O(n) |
+| 259 | [3Sum Smaller](https://leetcode.com/problems/3sum-smaller/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/259_3Sum_Smaller.py) | 1. Reduce to two sum smaller, then binary search, O(n^2lgn) and O(1)<br>2. Reduce to two sum smaller, then two points, O(n^2) and O(1)|
 | 337 | [House Robber III](https://leetcode.com/problems/house-robber-iii/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/337_House_Robber_III.py) | 1. Recursion with hash map, O(n) and O(n)<br>2. Recursion on two steps, O(n) and O(1) |
 | 340 | [Longest Substring with At Most K Distinct Characters](https://leetcode.com/problems/longest-substring-with-at-most-k-distinct-characters/) &hearts; | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/340_Longest_Substring_with_At_Most_K_Distinct_Characters.py) | Maintain a sliding window with at most k distinct characters and a count for this window. Note that the start position need a loop to update.|

python/259_3Sum_Smaller.py

@@ -0,0 +1,56 @@
+class Solution(object):
+    # def threeSumSmaller(self, nums, target):
+    #     """
+    #     :type nums: List[int]
+    #     :type target: int
+    #     :rtype: int
+    #     """
+    #     # https://leetcode.com/articles/3sum-smaller/#approach-2-binary-search-accepted
+    #     nums.sort()
+    #     ls = len(nums)
+    #     res = 0
+    #     for i in range(ls - 1):
+    #         res += self.twoSumSmaller(nums, i + 1, target - nums[i])
+    #     return res
+    #
+    # def twoSumSmaller(self, nums, start, target):
+    #     res = 0
+    #     for i in range(start, len(nums)):
+    #         pos = self.binarySearch(nums, i, target - nums[i])
+    #         res += pos - i
+    #     return res
+    #
+    # def binarySearch(self, nums, start, target):
+    #     left, right = start, len(nums) - 1
+    #     while left < right:
+    #         mid = (left + right + 1) / 2
+    #         if nums[mid] < target:
+    #             # left should always less than target
+    #             left = mid
+    #         else:
+    #             right = mid - 1
+    #     return left
+
+    def threeSumSmaller(self, nums, target):
+        """
+        :type nums: List[int]
+        :type target: int
+        :rtype: int
+        """
+        # https://leetcode.com/articles/3sum-smaller/#approach-2-binary-search-accepted
+        nums.sort()
+        ls = len(nums)
+        res = 0
+        for i in range(ls - 1):
+            res += self.twoSumSmaller(nums, i + 1, target - nums[i])
+        return res
+
+    def twoSumSmaller(self, nums, start, target):
+        res, left, right = 0, start, len(nums) - 1
+        while left < right:
+            if nums[left] + nums[right] < target:
+                res += right - left
+                left += 1
+            else:
+                right -= 1
+        return res