经典的Java函数实例
1. 阶乘函数
public static int factorial(int n) {
if (n == 0) {
return 1;
} else {
return n * factorial(n - 1);
}
}
2. Fibonacci数列函数
public static int fibonacci(int n) {
if (n == 0) {
return 0;
} else if (n == 1) {
return 1;
} else {
return fibonacci(n - 1) + fibonacci(n - 2);
}
}
3. 判断一个数是否为素数
public static boolean isPrime(int n) {
if (n <= 1) {
return false;
}
for (int i = 2; i <= Math.sqrt(n); i++) {
if (n % i == 0) {
return false;
}
}
return true;
}
4. 链表反转
public static ListNode reverseList(ListNode head) {
ListNode prev = null;
ListNode curr = head;
while (curr != null) {
ListNode next = curr.next;
curr.next = prev;
prev = curr;
curr = next;
}
return prev;
}
5. 逆波兰表达式求值
public static int evalRPN(String[] tokens) {
Stack<Integer> stack = new Stack<>();
for (String token : tokens) {
if (token.equals("+")) {
int b = stack.pop();
int a = stack.pop();
stack.push(a + b);
} else if (token.equals("-")) {
int b = stack.pop();
int a = stack.pop();
stack.push(a - b);
} else if (token.equals("*")) {
int b = stack.pop();
int a = stack.pop();
stack.push(a * b);
} else if (token.equals("/")) {
int b = stack.pop();
int a = stack.pop();
stack.push(a / b);
} else {
stack.push(Integer.parseInt(token));
}
}
return stack.pop();
}
6. 二分查找
public static int binarySearch(int[] arr, int target) {
int left = 0;
int right = arr.length - 1;
while (left <= right) {
int mid = left + (right - left) / 2;
if (arr[mid] == target) {
return mid;
} else if (arr[mid] < target) {
left = mid + 1;
} else {
right = mid - 1;
}
}
return -1;
}
7. 快速排序
public static void quickSort(int[] arr, int left, int right) {
if (left < right) {
int pivot = partition(arr, left, right);
quickSort(arr, left, pivot - 1);
quickSort(arr, pivot + 1, right);
}
}
private static int partition(int[] arr, int left, int right) {
int pivot = arr[right];
int i = left;
for (int j = left; j < right; j++) {
if (arr[j] < pivot) {
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
i++;
}
}
int temp = arr[i];
arr[i] = arr[right];
arr[right] = temp;
return i;
}
8. 归并排序
public static void mergeSort(int[] arr, int left, int right) {
if (left < right) {
int mid = (left + right) / 2;
mergeSort(arr, left, mid);
mergeSort(arr, mid + 1, right);
merge(arr, left, mid, right);
}
}
private static void merge(int[] arr, int left, int mid, int right) {
int[] temp = new int[right - left + 1];
int i = left;
int j = mid + 1;
int k = 0;
while (i <= mid && j <= right) {
if (arr[i] <= arr[j]) {
temp[k] = arr[i];
i++;
} else {
temp[k] = arr[j];
j++;
}
k++;
}
while (i <= mid) {
temp[k] = arr[i];
i++;
k++;
}
while (j <= right) {
temp[k] = arr[j];
j++;
k++;
}
for (int p = 0; p < temp.length; p++) {
arr[left + p] = temp[p];
}
}
9. 矩阵乘法
public static int[][] matrixMultiply(int[][] A, int[][] B) {
int m = A.length;
int n = B[0].length;
int[][] C = new int[m][n];
for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
for (int k = 0; k < B.length; k++) {
C[i][j] += A[i][k] * B[k][j];
}
}
}
return C;
}
10. 计算两个字符串的编辑距离
public static int minDistance(String word1, String word2) {
int m = word1.length();
int n = word2.length();
int[][] dp = new int[m + 1][n + 1];
for (int i = 1; i <= m; i++) {
dp[i][0] = i;
}
for (int j = 1; j <= n; j++) {
dp[0][j] = j;
}
for (int i = 1; i <= m; i++) {
for (int j = 1; j <= n; j++) {
if (word1.charAt(i - 1) == word2.charAt(j - 1)) {
dp[i][j] = dp[i - 1][j - 1];
} else {
dp[i][j] = Math.min(dp[i - 1][j - 1], Math.min(dp[i - 1][j], dp[i][j - 1])) + 1;
}
}
}
return dp[m][n];
}