提交部分easy题目

Author: MistRay

leetcode/src/main/java/com/mistray/array/BestTimeToBuyAndSellStock122.java

@@ -0,0 +1,24 @@
+package com.mistray.array;
+
+/**
+ * @author ZJY(MistRay)
+ * @Project algorithm-study
+ * @Package com.mistray.array
+ * @create 2020年05月21日 14:14
+ * @Desc
+ */
+public class BestTimeToBuyAndSellStock122 {
+
+    // 思路,贪心算法,大于就加上,小于就不管,最后就可以算出最大利润
+    public int maxProfit(int[] prices) {
+        int max = 0;
+        int pre = prices[0];
+        for (int price : prices) {
+            if (price - pre > 0){
+                max = max + price - pre;
+            }
+            pre = price;
+        }
+        return max;
+    }
+}

leetcode/src/main/java/com/mistray/array/ContainsDuplicate217.java

@@ -0,0 +1,42 @@
+package com.mistray.array;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.HashSet;
+
+/**
+ * @author ZJY(MistRay)
+ * @Project algorithm-study
+ * @Package com.mistray.array
+ * @create 2020年05月21日 15:08
+ * @Desc
+ */
+public class ContainsDuplicate217 {
+    // 思路 hashmap,hashset,排序
+//    public boolean containsDuplicate(int[] nums) {
+//        HashSet<Integer> set = new HashSet<>();
+//        for (int num : nums) {
+//            if (!set.contains(num)){
+//                set.add(num);
+//            }else{
+//                return true;
+//            }
+//        }
+//        return false;
+//    }
+
+    public boolean containsDuplicate(int[] nums) {
+        if (nums.length == 0) {
+            return false;
+        }
+        Arrays.sort(nums);
+        int pre = nums[0] + 1;
+        for (int num : nums) {
+            if (pre == num) {
+                return true;
+            }
+            pre = num;
+        }
+        return false;
+    }
+}

leetcode/src/main/java/com/mistray/array/MajorityElement169.java

@@ -0,0 +1,23 @@
+package com.mistray.array;
+
+import java.lang.reflect.Array;
+import java.util.Arrays;
+
+/**
+ * @author ZJY(MistRay)
+ * @Project algorithm-study
+ * @Package com.mistray.array
+ * @create 2020年05月21日 13:36
+ * @Desc
+ */
+public class MajorityElement169 {
+
+    // 思路:由于,如果数组中必然存在大于[n/2]个数的元素的话
+    // 可以先进行排序,取中间的数,必然是这个元素
+    // 例: 1 1 1 2
+    // 也可以使用hash的方式.
+    public int majorityElement(int[] nums) {
+        Arrays.sort(nums);
+        return  nums[nums.length/2];
+    }
+}

leetcode/src/main/java/com/mistray/link/DeleteNodeInALinkedList237.java

@@ -0,0 +1,20 @@
+package com.mistray.link;
+
+/**
+ * @author ZJY(MistRay)
+ * @Project algorithm-study
+ * @Package com.mistray.link
+ * @create 2020年05月21日 10:44
+ * @Desc
+ */
+public class DeleteNodeInALinkedList237 {
+
+    // 思路:因为无法访问上一个节点,所以要把当前节点变成下一个节点,然后把下一个节点干掉.
+    public void deleteNode(ListNode node) {
+        // 因为无法访问前一个结点，所以可以把要删除的结点的后一个结点的值前移
+        node.val = node.next.val;
+        // 然后删除掉后一个结点
+        node.next = node.next.next;
+    }
+
+}

leetcode/src/main/java/com/mistray/link/MergeTwoSortedLists21.java

@@ -18,15 +18,15 @@ public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
         if (l1 == null) {
             return l2;
         }
+
         if (l2 == null) {
             return l1;
         }
-
         if (l1.val > l2.val) {
             l2.next = mergeTwoLists(l2.next, l1);
             return l2;
         } else {
-            l1.next = mergeTwoLists(l2, l1.next);
+            l1.next = mergeTwoLists(l1.next, l2);
             return l1;
         }
     }

leetcode/src/main/java/com/mistray/stack/MinStack.java

@@ -0,0 +1,38 @@
+package com.mistray.stack;
+
+import java.util.Stack;
+
+class MinStack {
+
+    private Stack<Integer> dataStack;
+    private Stack<Integer> minStack;
+
+    // 思路:
+    // 使用辅助栈的方式,让取最小值的时间复杂度保持在O(1)
+    /** initialize your data structure here. */
+    public MinStack() {
+        dataStack = new Stack<>();
+        minStack = new Stack<>();
+    }
+
+    public void push(int x) {
+        dataStack.push(x);
+        if (minStack.isEmpty() || minStack.peek() >= x){
+            minStack.push(x);
+        }
+    }
+
+    public void pop() {
+        if (dataStack.pop().equals(minStack.peek()) && minStack.size() > 1){
+            minStack.pop();
+        }
+    }
+
+    public int top() {
+        return dataStack.peek();
+    }
+
+    public int getMin() {
+        return minStack.peek();
+    }
+}

leetcode/src/main/java/com/mistray/stack/MinStack155.java

@@ -0,0 +1,13 @@
+package com.mistray.stack;
+
+/**
+ * @author ZJY(MistRay)
+ * @Project algorithm-study
+ * @Package com.mistray.stack
+ * @create 2020年05月21日 14:33
+ * @Desc
+ */
+public class MinStack155 {
+
+
+}

leetcode/src/main/java/com/mistray/string/ReverseString242.java

@@ -0,0 +1,26 @@
+package com.mistray.string;
+
+/**
+ * @author ZJY(MistRay)
+ * @Project algorithm-study
+ * @Package com.mistray.string
+ * @create 2020年05月21日 11:09
+ * @Desc
+ */
+public class ReverseString242 {
+
+    public static void main(String[] args) {
+        char[] s= {'1','2','3'};
+        ReverseString242 reverseString242 = new ReverseString242();
+        reverseString242.reverseString(s);
+        System.out.println(s);
+    }
+
+    public void reverseString(char[] s) {
+        for (int i = 0, j = s.length - 1; i < s.length/2; i++, j--) {
+            char tmp = s[j];
+            s[j] = s[i];
+            s[i] = tmp;
+        }
+    }
+}

leetcode/src/main/java/com/mistray/string/ReverseWordsInAString557.java

@@ -0,0 +1,29 @@
+package com.mistray.string;
+
+/**
+ * @author ZJY(MistRay)
+ * @Project algorithm-study
+ * @Package com.mistray.string
+ * @create 2020年05月21日 10:57
+ * @Desc
+ */
+public class ReverseWordsInAString557 {
+
+    public static void main(String[] args) {
+        ReverseWordsInAString557 reverseWordsInAString557 = new ReverseWordsInAString557();
+        reverseWordsInAString557.reverseWords("12");
+    }
+
+    public String reverseWords(String s) {
+        String[] sp = s.split(" ");
+        StringBuilder sb = new StringBuilder();
+        for (String s1 : sp) {
+            char[] chars = s1.toCharArray();
+            for (int i = chars.length; i > 0; i--) {
+                sb.append(chars[i - 1]);
+            }
+            sb.append(" ");
+        }
+        return sb.toString().trim();
+    }
+}

leetcode/src/main/java/com/mistray/tree/LowestCommonAncestorOfABinarySearchTree235.java

@@ -0,0 +1,25 @@
+package com.mistray.tree;
+
+/**
+ * @author ZJY(MistRay)
+ * @Project algorithm-study
+ * @Package com.mistray.tree
+ * @create 2020年05月21日 13:22
+ * @Desc
+ */
+public class LowestCommonAncestorOfABinarySearchTree235 {
+
+    // 思路,根节点比两个点都大,去左树查找
+    // 根节点比两个都小,去右边查找
+    // 否则,直接返回根节点
+    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
+        if (root.val > p.val && root.val > q.val) {
+            return lowestCommonAncestor(root.left,p,q);
+        }
+        if (root.val < p.val && root.val < q.val) {
+            return lowestCommonAncestor(root.right,p,q);
+        }
+        return root;
+    }
+
+}