20190105

Author: mJackie

code/lc148.java

@@ -0,0 +1,55 @@
+package code;
+/*
+ * 148. Sort List
+ * 题意：链表排序
+ * 难度：Medium
+ * 分类：Linked List, Sort
+ * 思路：快慢指针把链表分成两半，在merge两个链表
+ * Tips：空间复杂度不是O(1)的，但是几个高票答案都是这样写的，面试给出这样的代码应该也够了
+ */
+public class lc148 {
+    public class ListNode {
+        int val;
+        ListNode next;
+        ListNode(int x) { val = x; }
+    }
+
+    public ListNode sortList(ListNode head) {
+        if( head==null || head.next == null ){
+            return head;
+        }
+        ListNode slow = head;
+        ListNode fast = head.next;
+        while( fast.next!=null && fast.next.next!=null ){   //把链表分成两半
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+        ListNode l2 = sortList(slow.next);
+        slow.next = null;
+        ListNode l1 = sortList(head);
+        return mergeList(l1, l2);
+    }
+
+    public ListNode mergeList(ListNode l1, ListNode l2){
+        ListNode res = new ListNode(0);
+        ListNode head = res;
+        while( l1!=null && l2!=null ){
+            if(l1.val<l2.val){
+                res.next = l1;
+                l1 = l1.next;
+                res = res.next;
+            }else{
+                res.next = l2;
+                l2 = l2.next;
+                res = res.next;
+            }
+        }
+        if(l1!=null){
+            res.next = l1;
+        }
+        if(l2!=null){
+            res.next = l2;
+        }
+        return head.next;
+    }
+}

code/lc152.java

@@ -0,0 +1,30 @@
+package code;
+/*
+ * 152. Maximum Product Subarray
+ * 题意：连续子序列最大乘积
+ * 难度：Medium
+ * 分类：Array, Dynamic Programming
+ * 思路：保存最大，最小值，因为负负得正。dp不用保存数组，空间可以压缩。
+ * Tips：Product是乘的意思
+ */
+public class lc152 {
+    public static void main(String[] args) {
+        int[] nums = {-4,-3,-2};
+        System.out.println(maxProduct(nums));
+    }
+    public static int maxProduct(int[] nums) {
+        int max = nums[0], min = nums[0], res = nums[0];
+        for (int i = 1; i < nums.length ; i++) {
+            if(nums[i]>0){
+                max = Math.max(nums[i],nums[i]*max);
+                min = Math.min(nums[i],nums[i]*min);
+            }else{
+                int temp = max; //注意maxh会被替换，先保存下
+                max = Math.max(nums[i],nums[i]*min);
+                min = Math.min(nums[i],nums[i]*temp);
+            }
+            res = Math.max(max, res);
+        }
+        return res;
+    }
+}

code/lc155.java

@@ -0,0 +1,44 @@
+package code;
+/*
+ * 155. Min Stack
+ * 题意：设计一个栈，这个栈有一个getmin方法
+ * 难度：Easy
+ * 分类：Stack, Design
+ * 思路：每次替换最小值时，把当前最小值入栈用以记录，以便之后更新最小值
+ * Tips：
+ */
+import java.util.Stack;
+
+public class lc155 {
+    class MinStack {
+        Stack<Integer> st;
+        int min = Integer.MAX_VALUE;
+        /** initialize your data structure here. */
+        public MinStack() {
+            this.st = new Stack<Integer>();
+        }
+
+        public void push(int x) {
+            if(x<=min){     //别忘了=
+                st.push(this.min);
+                this.min = x;
+            }
+            st.push(x);
+        }
+
+        public void pop() {
+            int x = st.pop();
+            if(x==this.min){
+                this.min = st.pop();
+            }
+        }
+
+        public int top() {
+            return st.peek();
+        }
+
+        public int getMin() {
+            return this.min;
+        }
+    }
+}

readme.md

@@ -76,9 +76,9 @@ Welcome to improve this project with me.*
 | 141  | [Linked List Cycle](https://leetcode.com/problems/linked-list-cycle/) | Easy | [Java](./code/lc141.java)
 | 142  | [Linked List Cycle II](https://leetcode.com/problems/linked-list-cycle-ii/) |Medium| [Java](./code/lc142.java)
 | 146  | [LRU Cache](https://leetcode.com/problems/lru-cache/)  | Hard | [Java](./code/lc146.java)
-| 148  | [Sort List](https://leetcode.com/problems/sort-list/)  |Medium| [Java]
-| 152  | [Maximum Product Subarray](https://leetcode.com/problems/maximum-product-subarray/) |Medium| [Java]
-| 155  | [Min Stack](https://leetcode.com/problems/min-stack/)  | Easy | [Java]
+| 148  | [Sort List](https://leetcode.com/problems/sort-list/)  |Medium| [Java](./code/lc148.java)
+| 152  | [Maximum Product Subarray](https://leetcode.com/problems/maximum-product-subarray/) |Medium| [Java](./code/lc152.java)
+| 155  | [Min Stack](https://leetcode.com/problems/min-stack/)  | Easy | [Java](./code/lc155.java)
 | 160  | [Intersection of Two Linked Lists](https://leetcode.com/problems/intersection-of-two-linked-lists/) | Easy | [Java]
 | 169  | [Majority Element](https://leetcode.com/problems/majority-element/) | Easy | [Java]
 | 198  | [House Robber](https://leetcode.com/problems/house-robber/)  | Easy | [Java]