solve problem Find The Duplicate Number

Author: P-ppc

README.md

@@ -258,6 +258,7 @@ All solutions will be accepted!
 |56|[Merge Intervals](https://leetcode-cn.com/problems/merge-intervals/description/)|[java/py/js](./algorithms/MergeIntervals)|Medium|
 |11|[Container With Most Water](https://leetcode-cn.com/problems/container-with-most-water/description/)|[java/py/js](./algorithms/ContainerWithMostWater)|Medium|
 |338|[Counting Bits](https://leetcode-cn.com/problems/counting-bits/description/)|[java/py/js](./algorithms/CountingBits)|Medium|
+|287|[Find The Duplicate Number](https://leetcode-cn.com/problems/find-the-duplicate-number/description/)|[java/py/js](./algorithms/FindTheDuplicateNumber)|Medium|
 
 # Database
 |#|Title|Solution|Difficulty|

algorithms/FindTheDuplicateNumber/README.md

@@ -0,0 +1,2 @@
+# Find The Duplicate Number
+We can solve this problem by Binary Search

algorithms/FindTheDuplicateNumber/Solution.java

@@ -0,0 +1,22 @@
+class Solution {
+    public int findDuplicate(int[] nums) {
+        int low = 1,
+            high = nums.length - 1;
+        
+        while (low < high) {
+            int mid = (low + high) / 2,
+                count = 0;
+            
+            for (int n : nums)
+                count += n <= mid ? 1 : 0;
+            
+            if (count > mid) {
+                high = mid;
+            } else {
+                low = mid + 1;
+            }
+        }
+        
+        return low;
+    }
+}

algorithms/FindTheDuplicateNumber/solution.js

@@ -0,0 +1,23 @@
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var findDuplicate = function(nums) {
+    let low = 1,
+        high = nums.length - 1
+    
+    while (low < high) {
+        let mid = parseInt((low + high) / 2),
+            count = 0
+        
+        nums.forEach(n => count += (n <= mid ? 1 : 0))
+        
+        if (count > mid) {
+            high = mid
+        } else {
+            low = mid + 1
+        }
+    }
+    
+    return low
+};

algorithms/FindTheDuplicateNumber/solution.py

@@ -0,0 +1,19 @@
+class Solution(object):
+    def findDuplicate(self, nums):
+        """
+        :type nums: List[int]
+        :rtype: int
+        """
+        low = 1
+        high = len(nums) - 1
+        
+        while low < high:
+            mid = (low + high) / 2
+            count = sum(x <= mid for x in nums)
+                    
+            if count > mid:
+                high = mid
+            else:
+                low = mid + 1
+        
+        return low