leetcode解题

125.valid-palindrome.java

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+/*
+ * @lc app=leetcode id=125 lang=java
+ *
+ * [125] Valid Palindrome
+ */
+
+// @lc code=start
+class Solution {
+    public boolean isPalindrome(String s) {
+        int i = 0, j = s.length()-1;
+        boolean r = true;
+        while(i < j) {
+            while(i<j && !isAlphanumberic(s.charAt(i))) {
+                i++;
+            }
+            while(i<j && !isAlphanumberic(s.charAt(j))) {
+                j--;
+            }
+            if(i==j) {
+                break;
+            }
+            if(!isSameLeterIgnoreCase(s.charAt(i), s.charAt(j))) {
+                r = false;
+                break;
+            }
+        }
+        return r;
+    }
+
+    private boolean isAlphanumberic(char c) {
+        return c >= 'a' && c <= 'z' || 
+        c >= 'A' && c <= 'Z' || c >='0' && c <= '9';
+    }
+
+    private boolean isSameLeterIgnoreCase(char a, char b) {
+        return a == b || a - b == 'z' - 'a' || a - b == 'a' - 'z';
+    }
+}
+// @lc code=end
+

136.single-number.java

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+/*
+ * @lc app=leetcode id=136 lang=java
+ *
+ * [136] Single Number
+ */
+
+// @lc code=start
+class Solution {
+    /**
+     * 可以通过对数组中所有整数进行异或来找出该数，
+     * 因为对于出现两次的整数，异或之后都为0，
+     * 而对于只出现过一次的整数，和0异或之后为其本身
+     * 
+     * @param nums
+     * @return
+     */
+    public int singleNumber(int[] nums) {
+        int r = 0;
+        for(int i = 0; i < nums.length; i++) {
+            r = r ^ nums[i];
+        }
+        return r;
+    }
+}
+// @lc code=end
+

141.linked-list-cycle.java

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+/*
+ * @lc app=leetcode id=141 lang=java
+ *
+ * [141] Linked List Cycle
+ */
+
+// @lc code=start
+/**
+ * Definition for singly-linked list.
+ * class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) {
+ *         val = x;
+ *         next = null;
+ *     }
+ * }
+ */
+public class Solution {
+    /**
+     * 快慢指针，若链表中存在环，则快指针一次动两步，慢指针一次动一步，快指针一定会赶上慢指针
+     * 
+     * @param head
+     * @return
+     */
+    public boolean hasCycle(ListNode head) {
+        ListNode slow = head, fast = head;
+        boolean isCycle = false;
+        while(fast != null && fast.next != null) {
+            fast = fast.next.next;
+            slow = slow.next;
+            if(fast == slow) {
+                isCycle = true;
+                break;
+            }
+        }
+        return isCycle;
+    }
+}
+// @lc code=end
+