阿华代码，不是逆风，就是我疯
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目录
一：前缀和模版
java
import java.util.Scanner;
// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
//1：获取输入
int n = in.nextInt() , q = in.nextInt();//长度为n的数组，q次查询
int[] arr = new int[n+1];
for(int i = 1 ; i <= n ; i++){
arr[i] = in.nextInt();
}
//2：创建dp数组
long[] dp = new long[n+1];
for(int i = 1 ; i <= n ; i++){
dp[i] = dp[i-1] + arr[i];
}
while(q > 0){
int l = in.nextInt() , r = in.nextInt();
System.out.println(dp[r]-dp[l-1]);
q--;
}
}
}二：前缀和模版2
java
import java.util.Scanner;
// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
// 注意 hasNext 和 hasNextLine 的区别
int n = in.nextInt() , m = in.nextInt() , q = in.nextInt();
//arr数组
long[][] arr = new long[n+1][m+1];
for(int i = 1 ; i <= n ; i++){
for(int j = 1 ; j <= m ; j++){
arr[i][j] = in.nextInt();
}
}
//copy创建一个dp数组
long[][] dp = new long[n+1][m+1];
for(int i = 1 ; i <= n ; i++){
for(int j = 1 ; j <= m ; j++){
dp[i][j] = dp[i-1][j] + dp[i][j-1] - dp[i-1][j-1] + arr[i][j];
}
}
while(q > 0){
int x1 = in.nextInt() , y1 = in.nextInt() , x2 = in.nextInt() , y2 = in.nextInt();
long result = dp[x2][y2] - dp[x2][y1-1] - dp[x1-1][y2] + dp[x1-1][y1-1];
System.out.println(result);
q--;
}
}
}三：寻找数组的中心下标
java
class Solution {
public int pivotIndex(int[] nums) {
int n = nums.length;
//前缀和数组
int[] f = new int[n];
f[0] = 0;
for(int i = 1 ; i < n ; i++){
f[i] = f[i-1] + nums[i-1];
}
//后缀和数组
int[] g = new int[n];
g[n-1] = 0;
for(int i = n-2 ; i >= 0 ; i--){
g[i] = g[i+1] + nums[i+1];
}
int result = Integer.MAX_VALUE;
for(int i = 0 ; i < n ; i++){
if(f[i] == g[i]){
result = Math.min(result,i);
}
}
if(result == Integer.MAX_VALUE){
return -1;
}else{
return result;
}
}
}四：除自身以外数组的乘积
238. 除自身以外数组的乘积 - 力扣（LeetCode）
java
class Solution {
public int[] productExceptSelf(int[] nums) {
int n = nums.length;
//前缀积
int[] f = new int[n];
f[0] = 1;
for(int i=1 ; i<n ; i++ ){
f[i] = f[i-1]*nums[i-1];
}
int[] g = new int[n];
g[n-1] = 1;
for(int i=n-2 ; i >= 0 ; i--){
g[i] = g[i+1]*nums[i+1];
}
int[] answer = new int[n];
for(int i = 0 ; i < n ; i++){
answer[i] = f[i]*g[i];
}
return answer;
}
}五：和为K的子数组
java
class Solution {
public int subarraySum(int[] nums, int k) {
Map<Integer,Integer> hash = new HashMap<>();
hash.put(0,1);
int sum = 0 , ret = 0;
for(int x : nums){
sum += x;
ret += hash.getOrDefault(sum-k,0);
hash.put(sum,hash.getOrDefault(sum,0)+1);
}
return ret;
}
}六：和被k整除的子数组
974. 和可被 K 整除的子数组 - 力扣（LeetCode）
java
class Solution {
public int subarraysDivByK(int[] nums, int k) {
Map<Integer,Integer> hashMap = new HashMap<Integer,Integer>();
hashMap.put(0 % k , 1);//默认有一个前缀和=0
int sum = 0;//前缀和
int ret = 0;//用来计数
for(int x : nums){
sum += x;
int remainder = (sum % k + k) % k;
ret += hashMap.getOrDefault(remainder,0);
hashMap.put(remainder,hashMap.getOrDefault(remainder,0)+1);
}
return ret;
}
}七：连续数组
java
class Solution {
public int findMaxLength(int[] nums) {
for(int i = 0 ; i < nums.length ; i++){
if(nums[i] == 0){
nums[i] = -1;
}
}
Map<Integer,Integer> hash = new HashMap<>();
hash.put(0,-1);
int sum = 0 , ret = 0;
for(int i = 0 ; i < nums.length ; i++){
sum += nums[i];
if(hash.containsKey(sum)){
int old = hash.get(sum);
int tem = i-old;
ret = Math.max(ret,tem);
}else{
hash.put(sum,i);
}
}
return ret;
}
}八：矩阵区域和
java
class Solution {
public int[][] matrixBlockSum(int[][] mat, int k) {
int m = mat.length , n = mat[0].length;
//1：初始化dp前缀和数组
int[][] dp = new int[m+1][n+1];
for(int i = 1 ; i < m+1 ; i++){
for(int j = 1 ; j < n+1 ; j++){
dp[i][j] = dp[i-1][j] + dp[i][j-1] -dp[i-1][j-1] + mat[i-1][j-1];
}
}
//2:处理前缀和数组
int[][] ret = new int[m][n];
for(int i = 0 ; i < m ; i++){
for(int j = 0 ; j < n ; j++){
int x1 = Math.max(0,i-k)+1 , y1 = Math.max(0,j-k)+1;
int x2 = Math.min(i+k,m-1)+1 , y2 = Math.min(j+k,n-1)+1;
ret[i][j] = dp[x2][y2] - dp[x2][y1-1] -dp[x1-1][y2] + dp[x1-1][y1-1];
}
}
return ret;
}
}