作者：互联网
20. 有效的括号
class Solution {
public boolean isValid(String s) {
Map<Character, Character> map = new HashMap<>() {
{
put(')', '(');
put('}', '{');
put(']', '[');
}
};
Deque<Character> stack = new LinkedList<>();
for(int i = 0; i < s.length(); i++) {
char ch = s.charAt(i);
//ch为左括号，直接入栈
if (!map.containsKey(ch)) {
stack.push(ch);
} else {
//ch为右括号，检查ch与栈顶元素是否配对
if (stack.isEmpty() || stack.pop() != map.get(ch)) {
return false;
}
}
}
//遍历完所有括号后，stack为空则说明有效
return stack.isEmpty();
}
}
21. 合并两个有序链表
迭代版：
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode() {}
* ListNode(int val) { this.val = val; }
* ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
ListNode dummyNode = new ListNode(-1);
ListNode temp = dummyNode;
while (l1 != null && l2 != null) {
if (l1.val < l2.val) {
temp.next = l1;
l1 = l1.next;
} else {
temp.next = l2;
l2 = l2.next;
}
temp = temp.next;
}
if (l1 != null) temp.next = l1;
if (l2 != null) temp.next = l2;
return dummyNode.next;
}
}
递归版：编写递归程序时一定不要用自己脑袋去模拟递归栈，而是要根据已知函数定义来写代码
class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
//base case
if (l1 == null) return l2;
if (l2 == null) return l1;
if (l1.val < l2.val) {
l1.next = mergeTwoLists(l1.next, l2);
return l1;
} else {
l2.next = mergeTwoLists(l1, l2.next);
return l2;
}
}
}
递归不理解的推荐题解：画解算法：21. 合并两个有序链表
22. 括号生成
思路：对于只有一种括号的情况，要生成合法括号字符串只需要满足：
-
左边的的括号数不多于括号对数
-
右边的括号数不多于左边的括号数
因此，只需要用回溯法生成所有组合，然后除去不符合条件的即可
class Solution {
public List<String> generateParenthesis(int n) {
char[] candidates = new char[] {'(', ')'};
StringBuilder track = new StringBuilder();
dfs(candidates, track, n, 0, 0);
return res;
}
private List<String> res = new LinkedList<>();
//left, right 分别记录当前track中左括号和右括号的数量
private void dfs(char[] candidates, StringBuilder track, int n, int left, int right) {
//去除不符合条件的结果，剪枝
if (left > n || right > left) return;
if (track.length() == 2*n) {
res.add(track.toString());
return;
}
for (int i = 0; i < candidates.length; i++) {
//选择
track.append(candidates[i]);
if (candidates[i] == '(') {
dfs(candidates, track, n, left + 1, right);
} else if (candidates[i] == ')') {
dfs(candidates, track, n, left, right + 1);
}
//撤销
track.deleteCharAt(track.length() - 1);
}
}
}
23. 合并K个升序链表
思路一：循环依次合并链表
思路二：两两归并
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode() {}
* ListNode(int val) { this.val = val; }
* ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
public ListNode mergeKLists(ListNode[] lists) {
if (lists == null || lists.length == 0) {
return null;
}
return mergeKLists(lists, 0, lists.length - 1);
}
//合并数组中下标在[left, right]之间的链表
private ListNode mergeKLists(ListNode[] lists, int left, int right) {
if (left == right) {
return lists[left];
}
int mid = left + (right - left) / 2;
ListNode leftList = mergeKLists(lists, left, mid);
ListNode rightList = mergeKLists(lists, mid + 1, right);
return mergeTwoList(leftList, rightList);
}
private ListNode mergeTwoList(ListNode l1, ListNode l2) {
ListNode dummyNode = new ListNode(-1);
ListNode temp = dummyNode;
while (l1 != null && l2 != null) {
if (l1.val < l2.val) {
temp.next = l1;
l1 = l1.next;
} else {
temp.next = l2;
l2 = l2.next;
}
temp = temp.next;
}
if (l1 != null) temp.next = l1;
if (l2 != null) temp.next = l2;
return dummyNode.next;
}
}
思路三：优先队列
class Solution {
public ListNode mergeKLists(ListNode[] lists) {
if (lists == null || lists.length == 0) {
return null;
}
//优先队列，小的链表结点位于队列前
Queue<ListNode> pq = new PriorityQueue<>((l1, l2) -> l1.val - l2.val);
for (int i = 0; i < lists.length; i++) {
if (lists[i] != null) {
pq.offer(lists[i]);
}
}
ListNode dummyNode = new ListNode(-1);
ListNode temp = dummyNode;
while (!pq.isEmpty()) {
ListNode minNode = pq.poll();
temp.next = minNode;
temp = minNode;
if (minNode.next != null) {
pq.offer(minNode.next);
}
}
return dummyNode.next;
}
}
31. 下一个排列
简单来说，对于数字排列来说字典顺序可以理解为升序。
class Solution {
public void nextPermutation(int[] nums) {
int len = nums.length;
//从右往左找找第一对升序数的位置，i指向较大元素
int i = len - 1;
while (i >= 1 && nums[i] <= nums[i - 1]) {
i--;
}
//存在升序对时：
//最坏情况下，交换升序对 nums[i - 1]、 nums[i]即可找到下一个排列;
//好点的情况下，升序对右侧（低位）可能存在 nums[k] 大于 nums[i - 1]，那么交换他们即可;
if (i >= 1) {
for (int k = len - 1; k >= i; k--) {
if (nums[k] > nums[i - 1]) {
swap(nums, k, i - 1);
break;
}
}
}
reverse(nums, i, len - 1);
}
private void swap(int[] nums, int i, int j) {
int temp = nums[i];
nums[i] = nums[j];
nums[j] = temp;
}
private void reverse(int[] nums, int i, int j) {
while (i < j) {
swap(nums, i, j);
i++;
j--;
}
}
}
32. 最长有效括号
思路：利用栈存储下标，方便计算有效括号子串长度。
- 首先将
-1入栈垫底 - 当字符为
'('，直接将下标入栈 - 当字符为
')'，弹出栈顶元素，若此时栈不为空，说明配对成功，然后用')'下标减去此时栈顶元素下标即为当前有效括号子串长度；若此时栈为空，说明未配对成功，直接将')'下标入栈垫底。
class Solution {
public int longestValidParentheses(String s) {
Deque<Integer> stack = new LinkedList<>();
stack.push(-1);
int maxLen = 0;
for (int i = 0; i < s.length(); i++) {
if (s.charAt(i) == '(') {
stack.push(i);
} else if (s.charAt(i) == ')'){
stack.pop();
// ) 配对成功
if (!stack.isEmpty()) {
maxLen = Math.max(maxLen, i - stack.peek());
// ) 配对失败
} else {
stack.push(i);
}
}
}
return maxLen;
}
}
33. 搜索旋转排序数组
思路：可知旋转后的数组分为前后两段，都为升序，且前面一段始终大于后面一段。可以利用二分法，始终拿 nums[mid] 和 nums[right] 比较然后缩小范围，具体如下：
- 若
nums[mid]小于nums[right]， 说明 mid 位于后半段，那么nums[mid, right]有序； - 若
nums[mid]大于nums[right]， 说明 mid 位于前半段，那么nums[left, mid]有序。找到有序区间后可以根据target值快速缩小区间。
class Solution {
public int search(int[] nums, int target) {
int left = 0;
int right = nums.length - 1;
//跳出循环时，left、 right 相邻，且都可能为target
while (left + 1 < right) {
int mid = left + (right - left) / 2;
//找到了
if (nums[mid] == target) {
return mid;
// mid 位于后半段上升段
} else if (nums[mid] < nums[right]) {
// target 位于[mid, right]之间
if (target >= nums[mid] && target <= nums[right]) {
left = mid;
} else {
right = mid;
}
// mid 位于前半段上升段
} else if (nums[mid] > nums[right]) {
// target 位于 [left, mid]之间
if (target >= nums[left] && target <= nums[mid]) {
right = mid;
} else {
left = mid;
}
}
// 由于 left + 1 < right，且nums不包含重复数，故不可能出现nums[mid] == nums[right]的情况
}
if (nums[left] == target) return left;
if (nums[right] == target) return right;
return -1;
}
}
34. 在排序数组中查找元素的第一个和最后一个位置
思路：带边界二分查找，利用 while (left + 1 < right)容易处理边界条件。
class Solution {
public int[] searchRange(int[] nums, int target) {
if (nums.length <= 0) return new int[] {-1, -1};
//左边界
int left = searchBorder(nums, target, Border.LEFT);
if (left == -1) return new int[] {-1, -1};
//右边界
int right = searchBorder(nums, target, Border.RIGHT);
return new int[] {left, right};
}
private int searchBorder(int[] nums, int target, Border border) {
int left = 0;
int right = nums.length - 1;
//跳出循环时，left、 right 相邻，且都可能为target
while (left + 1 < right) {
int mid = left + (right - left) / 2;
if (nums[mid] == target) {
//查找左边界
if (border == Border.LEFT) {
right = mid;
//查找右边界
} else if (border == Border.RIGHT) {
left = mid;
}
} else if (nums[mid] > target) {
right = mid;
} else if (nums[mid] < target) {
left = mid;
}
}
//没找到
if (nums[left] != target && nums[right] != target) {
return -1;
}
//查找左边界，优先返回左边元素
if (border == Border.LEFT) {
return nums[left] == target ? left : right;
//查找右边界，优先返回右边元素
} else if (border == Border.RIGHT) {
return nums[right] == target ? right : left;
} else {
throw new IllegalArgumentException("非法选项");
}
}
}
//使用枚举代替静态变量
enum Border {
LEFT, RIGHT
}
39. 组合总和
思路：回溯法
class Solution {
public List<List<Integer>> combinationSum(int[] candidates, int target) {
LinkedList<Integer> track = new LinkedList<>();
Arrays.sort(candidates); //排序后可实现进一步剪枝
dfs(candidates, track, target, 0);
return res;
}
private List<List<Integer>> res = new LinkedList<>();
private void dfs(int[] candidates, LinkedList<Integer> track, int target, int start) {
//不会再出现target小于0的情况
//base case
if (target == 0) {
res.add(new LinkedList<>(track));
return;
}
//通过start改变可选列表
for (int i = start; i < candidates.length; i++) {
//由于此时candidates是有序的，candidates[i] 后面的元素都大于target，直接忽略
if (target - candidates[i] < 0) {
break;
}
track.offerLast(candidates[i]);
dfs(candidates, track, target - candidates[i], i);
track.pollLast();
}
}
}
推荐题解：回溯算法 + 剪枝（回溯经典例题详解）
42. 接雨水
推荐一种不使用单调栈而是使用备忘录的解法：
思路一：在左边找大于等于当前高度的最大值，右边也找大于等于当前高度的最大值，两者取最小值再减去当前高度即为当前下标所能接的雨水量。
class Solution {
public int trap(int[] height) {
if (height == null || height.length <= 2) {
return 0;
}
int len = height.length;
//分别记录元素左边和右边的最大值
int[] leftMax = new int[len];
int[] rightMax = new int[len];
//最左边元素左边的最大值
leftMax[0] = height[0];
//最右边元素右边的最大值
rightMax[len - 1] = height[len - 1];
for (int i = 1; i < len; i++) {
leftMax[i] = Math.max(leftMax[i - 1], height[i]);
}
for (int i = len - 2; i >= 0; i--) {
rightMax[i] = Math.max(rightMax[i + 1], height[i]);
}
int area = 0;
for (int i = 1; i < len - 1; i++) {
area += Math.min(leftMax[i], rightMax[i]) - height[i];
}
return area;
}
}
思路二：单调栈
class Solution {
public int trap(int[] height) {
if (height == null || height.length <= 2) {
return 0;
}
//用来存储元素下标
Deque<Integer> stack = new LinkedList<>();
int area = 0;
for (int i = 0; i < height.length; i++) {
while(!stack.isEmpty() && height[i] > height[stack.peek()]) {
int top = stack.pop();
if (stack.isEmpty()) {
break;
}
int width = i - stack.peek() - 1;
int high = Math.min(height[stack.peek()], height[i]) - height[top];
area += width * high;
}
//入栈
stack.push(i);
}
return area;
}
}
推荐图解帮助理解：【接雨水】单调递减栈，简洁代码，动图模拟
标签：11,right,ListNode,nums,int,next,20,热题,left 来源： https://www.cnblogs.com/winlsr/p/14952178.html