做的一些题的解题思路
Product of Array Except Self
除本身之外的数组之积
Given an array of n integers where n > 1, nums, return an array output such that output[i] is equal to the product of all the elements of nums except nums[i].
Solve it without division and in O(n).
For example, given [1,2,3,4], return [24,12,8,6].
解题思路：拆分法
[A_234 , A_134 , A_124 , A_123 ]=
[1 , A_1 , A_12 , A_123 ]*
[A_234 , A_34 , A_4 , 1 ]
/**
* Created by Administrator on 2017/5/8.
*/
public class LeetCode {
public static void main(String[] args) {
// int [] nums={5, 7, 1, 8,3, 10}; //测试
int[] nums = {1, 3, 5, 6};
int k = 5;
int [] res = productExceptSelf(nums);
for (int i=0;i<res.length;i++) {
System.out.print(res[i]+" ");
}
}
public static int[] productExceptSelf(int[] nums) {
final int [] result = new int [nums.length];
final int [] right = new int [nums.length];
final int [] left = new int [nums.length];
left[0]=1;
for(int i=1;i<nums.length;i++){
left[i]=left[i-1]*nums[i-1];
}
right[nums.length-1]=1;
for(int i=nums.length-2;i>=0;i--){
right[i]=right[i+1]*nums[i+1];
}
for (int i=0;i<nums.length;i++){
result[i]=right[i]*left[i];
}
return result;
}
}
# Increasing Triplet Subsequence
Given an unsorted array return whether an increasing subsequence of length 3 exists or not in the array.
Formally the function should:
Return true if there exists i, j, k
such that arr[i] < arr[j] < arr[k] given 0 ≤ i < j < k ≤ n-1 else return false.
Your algorithm should run in O(n) time complexity and O(1) space complexity.
Examples:
Given [1, 2, 3, 4, 5],
return true.
Given [5, 4, 3, 2, 1],
return false.
用整数最大值去比较，x1记录第一个数，x2记录第二大的数，当出现第三大的数，则return true。
public class Solution {
public boolean increasingTriplet(int[] nums) {
int x1=Integer.MAX_VALUE;
int x2=Integer.MAX_VALUE;
for(int x: nums){
if(x<=x1) x1=x;
else if(x<=x2) x2=x;
else return true;
}
return false;
}
}
# Contains Duplicate II Given an array of integers and an integer k, find out whether there are two distinct indices i and j in the array such that nums[i] = nums[j] and the absolute difference between i and j is at most k.
维护一个HashMap,key为整数，value为下标，将数组中的元素不断添加到这个Hashmap中，遇到重复时，计算下标距离；
用Integer.MAX_VALUE 设置为比较的初始值；
学会用HashMap是非常关键的。
public class LeetCode {
public static void main(String[] args) {
int[] nums = {1, 3, 5, 1,6};
int k=3;
System.out.print(containsNearbyDuplicate(nums,k));
}
public static boolean containsNearbyDuplicate(int[] nums, int k) {
final Map<Integer,Integer> map = new HashMap<>();
int min=Integer.MAX_VALUE;
for(int i=0;i<nums.length;i++){
if(map.containsKey(nums[i])){
final int preIndex=map.get(nums[i]);
final int gap = i-preIndex;
min = Math.min(min,gap);
}
map.put(nums[i],i);
}
return min<=k;
}
}
# Add Two Numbers You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
主要学习怎么创建链表，怎么定义链表
public class LeetCode {
public static void main(String[] args) {
int[] inputl1=new int[]{2,4,3};
int[] inputl2=new int[]{5,6,4};
ListNode l1=buildListNode(inputl1);
ListNode l2=buildListNode(inputl2);
ListNode listNode =addTwoNumbers(l1,l2);
while(listNode!=null){
System.out.println("val "+listNode.val);
listNode=listNode.next;
}
}
//定义链表
public static class ListNode{
int val;
ListNode next;
ListNode(int val){
this.val=val;
this.next=null;
}
}
//创建链表
private static ListNode buildListNode(int[] input){
ListNode first = null,last = null,newNode;
int num;
if(input.length>0){
for(int i=0;i<input.length;i++){
newNode=new ListNode(input[i]);
newNode.next=null;
if(first==null){
first=newNode;
last=newNode;
}
else{
last.next=newNode;
last=newNode;
}
}
}
return first;
}
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode dummy =new ListNode(-1);
int carry = 0;
ListNode prev = dummy;
for(ListNode pa=l1 , pb=l2 ; pa!=null || pb!=null ;
pa=pa==null?null : pa.next,
pb=pb==null ? null : pb.next,
prev=prev.next){
final int ai=pa==null?0:pa.val;
final int bi=pb==null?0:pb.val;
final int value=(ai+bi+carry)%10;
carry=(ai+bi+carry)/10;
prev.next=new ListNode (value);
}
if(carry>0)
prev.next=new ListNode (carry);
return dummy.next;
}
}
Evaluate Reverse Polish Notation
计算逆波兰表达式（又叫后缀表达式）的值
'' 2 '','' 1 '','' + '', ''3'', ''* '' -->(2+1)*3-->9
用堆栈遇到运算符则把前面两个拿出来运算
public class Main {
public static void main(String[] args) {
String []tokens={"2", "1", "+", "3", "*"};
System.out.print(evalRPN(tokens));
}
public static int evalRPN(String [] tokens){
Stack<String> s = new Stack<>();
if(tokens.length==1){
return Integer.parseInt(tokens[0]);
}
for(String token:tokens){
if(!isOperator(token)){
s.push(token);
}else {
int y=Integer.parseInt(s.pop());
int x=Integer.parseInt(s.pop());
switch (token.charAt(0)){
case '+':x+=y;break;
case '-':x-=y;break;
case '*':x*=y;break;
case '/':x/=y;break;
}
s.push(String.valueOf(x));
}
}
return Integer.parseInt(s.peek());
}
private static boolean isOperator(final String op){
return op.length() == 1 && OPS.indexOf(op)!=-1;
}
private static String OPS = new String("+-*/");
}