Solve #287

Author: aylei

src/lib.rs

@@ -217,3 +217,4 @@ mod n0275_h_index_ii;
 mod n0279_perfect_squares;
 mod n0282_expression_add_operators;
 mod n0283_move_zeroes;
+mod n0287_find_the_duplicate_number;

src/n0287_find_the_duplicate_number.rs

@@ -0,0 +1,71 @@
+/**
+ * [287] Find the Duplicate Number
+ *
+ * Given an array nums containing n + 1 integers where each integer is between 1 and n (inclusive), prove that at least one duplicate number must exist. Assume that there is only one duplicate number, find the duplicate one.
+ *
+ * Example 1:
+ *
+ *
+ * Input: [1,3,4,2,2]
+ * Output: 2
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: [3,1,3,4,2]
+ * Output: 3
+ *
+ * Note:
+ *
+ * <ol>
+ * 	You must not modify the array (assume the array is read only).
+ * 	You must use only constant, O(1) extra space.
+ * 	Your runtime complexity should be less than O(n^2).
+ * 	There is only one duplicate number in the array, but it could be repeated more than once.
+ * </ol>
+ *
+ */
+pub struct Solution {}
+
+// submission codes start here
+
+// 假如把值看做 next node 的下标, 那么:
+//  从 0 出发不会回到 0
+//  一定有环, 因为 1-n 全部落在下标范围 [0, n] 中
+//  从 0 遍历经过的环中, 一定存在重复数字 x, 且 x 就是入环点的下标:
+//   1.从 0 走到入环点, 入环点的前驱值为 x; 2.入环点在环上的前驱值也是 x
+//   由于我们不可能回到 0, 因此这两个节点下标不同, x 即为要找的重复数字
+impl Solution {
+    pub fn find_duplicate(nums: Vec<i32>) -> i32 {
+        let mut slow: usize = nums[0] as usize;
+        let mut fast: usize = nums[nums[0] as usize] as usize;
+        // util slow meet fast
+        while slow != fast {
+            slow = nums[slow] as usize;
+            fast = nums[nums[fast] as usize] as usize;
+        }
+
+        fast = 0_usize;
+        while slow != fast {
+            fast = nums[fast] as usize;
+            slow = nums[slow] as usize;
+        }
+        slow as i32
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_287() {
+        assert_eq!(Solution::find_duplicate(vec![1,3,4,2,2]), 2);
+        assert_eq!(Solution::find_duplicate(vec![3,1,3,4,2]), 3);
+        assert_eq!(Solution::find_duplicate(vec![1,2,3,4,5,5]), 5);
+        assert_eq!(Solution::find_duplicate(vec![5,1,2,3,4,5]), 5);
+    }
+}