新增 LeetCode 题目解答: 206, 287, 225, 28, 2487, 15, 202

src/main/java/cn/whaifree/redo/redo_all_240721/LeetCode15.java

@@ -0,0 +1,58 @@
+package cn.whaifree.redo.redo_all_240721;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/7/23 22:46
+ * @注释
+ */
+public class LeetCode15 {
+    public static void main(String[] args) {
+        int[] nums = new int[]{-1, 0, 1, 2, -1, -4};
+        System.out.println(threeSum(nums));
+
+        System.out.println(threeSum(new int[]{1,2,1}));
+    }
+
+    public static List<List<Integer>> threeSum(int[] nums) {
+        int len = nums.length;
+        ArrayList<List<Integer>> res = new ArrayList<>();
+        if (len < 3) {
+            return res;
+        }
+        Arrays.sort(nums);
+
+
+        for (int left = 0; left < len - 2; left++) {
+            int mid = left + 1;
+            int right = len - 1;
+            if (left > 0 && nums[left] == nums[left - 1]) {
+                break;
+            }
+            while (mid < right) {
+                int sum = nums[left] + nums[mid] + nums[right];
+                if (sum == 0) {
+                    res.add(Arrays.asList(nums[left], nums[mid], nums[right]));
+                    while (mid < right && nums[mid] == nums[mid + 1]) {
+                        mid++;
+                    }
+                    while (mid < right && nums[right] == nums[right - 1]) {
+                        right--;
+                    }
+                    // 如果有重复的，必然要跳过
+                    mid++;
+                    right--;
+                } else if (sum > 0) {
+                    right--;
+                } else {
+                    mid++;
+                }
+            }
+        }
+        return res;
+    }
+}

src/main/java/cn/whaifree/redo/redo_all_240721/LeetCode202.java

@@ -0,0 +1,46 @@
+package cn.whaifree.redo.redo_all_240721;
+
+import java.util.HashSet;
+import java.util.Set;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/7/22 23:18
+ * @注释
+ */
+public class LeetCode202 {
+    public static void main(String[] args)
+    {
+        int n = 2;
+        System.out.println(isHappy(n));
+        System.out.println(isHappy(19));
+    }
+
+    /**
+     * 2 4 16 1+36=37 9+47=56 25+36=61 36+1=37
+     * @param n
+     * @return
+     */
+    public static boolean isHappy(int n)
+    {
+        Set<Integer> set = new HashSet<>();
+
+        while (n != 1) {
+            int tmpSum = 0;
+            while (n > 0) {
+                int retail = n % 10;
+                tmpSum += retail * retail;
+                n /= 10;
+            }
+            if (set.contains(tmpSum)) {
+                return false;
+            }
+            set.add(tmpSum);
+            n = tmpSum;
+        }
+        return true;
+
+
+    }
+}

src/main/java/cn/whaifree/redo/redo_all_240721/LeetCode206.java

@@ -0,0 +1,43 @@
+package cn.whaifree.redo.redo_all_240721;
+
+import cn.whaifree.leetCode.model.ListNode;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/7/22 22:56
+ * @注释
+ */
+public class LeetCode206 {
+    public static void main(String[] args)
+    {
+        ListNode listNode = ListNode.listNodeFromArray(new int[]{1, 2, 3, 4, 5});
+        ListNode.printList(reverseList(listNode));
+
+    }
+
+    /**
+     * 递归
+     * @param head
+     * @return
+     */
+    public static ListNode reverseList(ListNode head)
+    {
+        return reverse(null, head);
+    }
+
+    /**
+     *  a b c d
+     * @param pre
+     * @param after
+     * @return
+     */
+    public static ListNode reverse(ListNode pre, ListNode after) {
+        if (after == null) {
+            return pre;
+        }
+        ListNode tmp = after.next;
+        after.next = pre;
+        return reverse(after, tmp);
+    }
+}

src/main/java/cn/whaifree/redo/redo_all_240721/LeetCode225.java

@@ -0,0 +1,67 @@
+package cn.whaifree.redo.redo_all_240721;
+
+import java.util.Deque;
+import java.util.LinkedList;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/7/23 23:43
+ * @注释
+ */
+public class LeetCode225 {
+    public static void main(String[] args)
+    {
+        MyStack myStack = new MyStack();
+        myStack.push(1);
+        myStack.push(2);
+        System.out.println(myStack.top());
+        System.out.println(myStack.pop());
+        System.out.println(myStack.empty());
+        System.out.println(myStack.pop());
+        System.out.println(myStack.empty());
+    }
+
+    static class MyStack {
+
+        Deque<Integer> q1;
+        Deque<Integer> q2;
+
+        public MyStack() {
+            q1 = new LinkedList<Integer>();
+            q2 = new LinkedList<Integer>();
+        }
+
+        public void push(int x) {
+            q1.add(x);
+        }
+
+        public int pop() {
+            while (q1.size() > 1) {
+                q2.add(q1.pop());
+            }
+            int tmp = q1.pop();
+            while (!q2.isEmpty()) {
+                q1.add(q2.pop());
+            }
+            return tmp;
+        }
+
+        public int top() {
+            while (q1.size() > 1) {
+                q2.add(q1.pop());
+            }
+            int tmp = q1.pop();
+            q2.add(tmp);
+            while (!q2.isEmpty()) {
+                q1.add(q2.pop());
+            }
+            return tmp;
+        }
+
+        public boolean empty() {
+            return q1.isEmpty();
+        }
+    }
+
+}

src/main/java/cn/whaifree/redo/redo_all_240721/LeetCode2487.java

@@ -0,0 +1,49 @@
+package cn.whaifree.redo.redo_all_240721;
+
+import cn.whaifree.leetCode.model.ListNode;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/7/23 23:09
+ * @注释
+ */
+public class LeetCode2487 {
+    public static void main(String[] args) {
+        ListNode listNode = ListNode.listNodeFromArray(new int[]{99, 2, 13, 3, 8});
+        removeNodes(listNode).printList();
+    }
+
+    public static ListNode removeNodes(ListNode head) {
+        head = reverseList(new ListNode(-1, head));
+        ListNode index = head;
+        while (index.next != null) {
+            if (index.val > index.next.val) {
+                index.next = index.next.next;
+                continue;
+            }
+            index = index.next;
+        }
+        return reverseList(head);
+    }
+
+    public static ListNode reverseList(ListNode head) {
+        return reverse(null, head);
+    }
+
+    /**
+     * a b c
+     * @param pre
+     * @param after
+     * @return
+     */
+    public static ListNode reverse(ListNode pre, ListNode after) {
+        if (after == null) {
+            return pre;
+        }
+        ListNode tmp = after.next;
+        after.next = pre;
+        return reverse(after, tmp);
+    }
+
+}

src/main/java/cn/whaifree/redo/redo_all_240721/LeetCode28.java

@@ -0,0 +1,29 @@
+package cn.whaifree.redo.redo_all_240721;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/7/23 23:37
+ * @注释
+ */
+public class LeetCode28 {
+    public static void main(String[] args)
+    {
+        String haystack = "hello";
+        String needle = "ll";
+        System.out.println(strStr(haystack, needle));
+        System.out.println(strStr("a", "a"));
+        System.out.println(strStr("mississippi", "issip"));
+    }
+    public static int strStr(String haystack, String needle)
+    {
+        int len = needle.length();
+        for (int i = 0; i <= haystack.length() - len; i++) {
+            String substring = haystack.substring(i, i + len);
+            if (substring.equals(needle)) {
+                return i;
+            }
+        }
+        return -1;
+    }
+}

src/main/java/cn/whaifree/redo/redo_all_240721/LeetCode287.java

@@ -0,0 +1,52 @@
+package cn.whaifree.redo.redo_all_240721;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/7/22 23:12
+ * @注释
+ */
+public class LeetCode287
+{
+    public static void main(String[] args)
+    {
+        int[] nums = {3,1,3,4,2};
+        System.out.println(findDuplicate(nums));
+        int[] nums2 = {1,3,4,2,2};
+        System.out.println(findDuplicate(nums2));
+        int[] nums3 = {1,1};
+        System.out.println(findDuplicate(nums3));
+    }
+
+    /**
+     * 包含 n + 1 个整数的数组 nums ，其数字都在 [1, n] 范围内（包括 1 和 n）
+     *
+     * 3,1,3,4,2
+     * 0 1 2 3 4
+     *
+     * fast = nums[nums[fast]]
+     * slow = nums[slow]
+     *
+     *
+     * @param nums
+     * @return
+     */
+    public static int findDuplicate(int[] nums)
+    {
+        // 快慢指针
+        int fast = 0;
+        int slow = 0;
+        do {
+            fast = nums[nums[fast]];
+            slow = nums[slow];
+        } while (slow != fast);
+
+        fast = 0;
+
+        while (fast != slow) {
+            fast = nums[fast];
+            slow = nums[slow];
+        }
+        return slow;
+    }
+}