【LeetCode】图解算法数据结构+java代码实现（数据结构篇）

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一、替换空格

    public String replaceSpace(String s) {
        StringBuilder stringBuilder = new StringBuilder();
        for (char c : s.toCharArray()) {
            if(c == ' '){
                stringBuilder.append("%20");
            }else{
                stringBuilder.append(c);
            }
        }
        return stringBuilder.toString();
    }
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二、从尾到头打印链表

	public int[] reversePrint(ListNode head) {
        return dfs(head,0);
    }

    public int[] dfs(ListNode head,int cnt){
        if(head == null){
            return new int[cnt];
        }
        int[] arr = dfs(head.next, cnt + 1);
        arr[arr.length - cnt - 1] = head.val;
        return arr;
    }

    public class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
        }
    }
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三、用两个栈实现队列

法一：LinkedList模拟

	class CQueue {
        LinkedList<Integer> A, B;
        public CQueue() {
            A = new LinkedList<Integer>();
            B = new LinkedList<Integer>();
        }
        public void appendTail(int value) {
            A.addLast(value);
        }
        public int deleteHead() {
            if(!B.isEmpty()) {
                return B.removeLast();
            }
            if(A.isEmpty()) {
                return -1;
            }
            while(!A.isEmpty()) {
                B.addLast(A.removeLast());
            }
            return B.removeLast();
        }
    }
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法二：用原生的Queue

	class CQueue{
        Queue<Integer> queue = new ArrayDeque<>();
        public void appendTail(int value) {
            queue.add(value);
        }
        public int deleteHead() {
            if(queue.isEmpty()){
                return -1;
            }
            return queue.poll();
        }
    }
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四、表示数值的字符串

本题使用有限状态自动机。根据字符类型和合法数值的特点，先定义状态，再画出状态转移图，最后编写代码即可。

	public boolean isNumber(String s) {
        Map[] states = {
                new HashMap<Character,Integer>() {{ put(' ', 0); put('s', 1); put('d', 2); put('.', 4); }}, // 0.
                new HashMap<Character,Integer>() {{ put('d', 2); put('.', 4); }},                           // 1.
                new HashMap<Character,Integer>() {{ put('d', 2); put('.', 3); put('e', 5); put(' ', 8); }}, // 2.
                new HashMap<Character,Integer>() {{ put('d', 3); put('e', 5); put(' ', 8); }},              // 3.
                new HashMap<Character,Integer>() {{ put('d', 3); }},                                        // 4.
                new HashMap<Character,Integer>() {{ put('s', 6); put('d', 7); }},                           // 5.
                new HashMap<Character,Integer>() {{ put('d', 7); }},                                        // 6.
                new HashMap<Character,Integer>() {{ put('d', 7); put(' ', 8); }},                           // 7.
                new HashMap<Character,Integer>() {{ put(' ', 8); }}                                         // 8.
        };
        int p = 0;
        char t;
        for(char c : s.toCharArray()) {
            if(c >= '0' && c <= '9') {
                t = 'd';
            } else if(c == '+' || c == '-') {
                t = 's';
            } else if(c == 'e' || c == 'E') {
                t = 'e';
            } else if(c == '.' || c == ' ') {
                t = c;
            } else {
                t = '?';
            }
            if(!states[p].containsKey(t)) {
                return false;
            }
            p = (int)states[p].get(t);
        }
        return p == 2 || p == 3 || p == 7 || p == 8;
    }
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五、反转链表

	ListNode node;
    public ListNode reverseList(ListNode head) {
        if(head == null){
            return null;
        }
        dfs(head);
        return node;
    }

    private ListNode dfs(ListNode head) {
        if(head.next == null){
            node =  new ListNode(head.val);
            return node;
        }
        ListNode listNode = dfs(head.next);
        listNode.next = new ListNode(head.val);
        return listNode.next;
    }

    public class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
        }
    }
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六、包含 min 函数的栈

	class MinStack {

        Stack<Integer> stack = new Stack<>();
        Stack<Integer> minStack = new Stack<>();

        /** initialize your data structure here. */
        public MinStack() {

        }

        public void push(int x) {
            if(minStack.isEmpty() || minStack.peek() >= x){
                minStack.add(x);
            }
            stack.add(x);
        }

        public void pop() {
            int pop = stack.pop();
            if(!minStack.isEmpty() && pop == minStack.peek()){
                minStack.pop();
            }
        }

        public int top() {
            return stack.peek();
        }

        public int min() {
            if(minStack.isEmpty()){
                return -1;
            }
            return minStack.peek();
        }
    }
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七、复杂链表的复制

	public Node copyRandomList(Node head) {
        if(head == null) {
            return null;
        }
        Node cur = head;
        Map<Node, Node> map = new HashMap<>();
        // 3. 复制各节点，并建立 “原节点 -> 新节点” 的 Map 映射
        while(cur != null) {
            map.put(cur, new Node(cur.val));
            cur = cur.next;
        }
        cur = head;
        // 4. 构建新链表的 next 和 random 指向
        while(cur != null) {
            map.get(cur).next = map.get(cur.next);
            map.get(cur).random = map.get(cur.random);
            cur = cur.next;
        }
        // 5. 返回新链表的头节点
        return map.get(head);
    }

    class Node {
        int val;
        Node next;
        Node random;

        public Node(int val) {
            this.val = val;
            this.next = null;
            this.random = null;
        }
    }
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八、II. 左旋转字符串

    public String reverseLeftWords(String s, int n) {
        StringBuilder stringBuilder = new StringBuilder();
        char[] chars = s.toCharArray();
        for (int i = n; i < chars.length; i++) {
            stringBuilder.append(chars[i]);
        }
        for (int i = 0; i < n; i++) {
            stringBuilder.append(chars[i]);
        }
        return stringBuilder.toString();
    }
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九、I. 滑动窗口的最大值

	public int[] maxSlidingWindow(int[] nums, int k) {
        if(nums.length == 0 || k == 0) {
            return new int[0];
        }
        Deque<Integer> deque = new LinkedList<>();
        int[] res = new int[nums.length - k + 1];
        for(int j = 0, i = 1 - k; j < nums.length; i++, j++) {
            // 删除 deque 中对应的 nums[i-1]
            if(i > 0 && deque.peekFirst() == nums[i - 1]) {
                deque.removeFirst();
            }
            // 保持 deque 递减
            while(!deque.isEmpty() && deque.peekLast() < nums[j]) {
                deque.removeLast();
            }
            deque.addLast(nums[j]);
            // 记录窗口最大值
            if(i >= 0) {
                res[i] = deque.peekFirst();
            }
        }
        return res;
    }
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十、II. 队列的最大值

    class MaxQueue {
        Queue<Integer> queue;
        Deque<Integer> deque;
        public MaxQueue() {
            queue = new LinkedList<>();
            deque = new LinkedList<>();
        }
        public int max_value() {
            return deque.isEmpty() ? -1 : deque.peekFirst();
        }
        public void push_back(int value) {
            queue.offer(value);
            while(!deque.isEmpty() && deque.peekLast() < value) {
                deque.pollLast();
            }
            deque.offerLast(value);
        }
        public int pop_front() {
            if(queue.isEmpty()) {
                return -1;
            }
            if(queue.peek().equals(deque.peekFirst())) {
                deque.pollFirst();
            }
            return queue.poll();
        }
    }
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十一、把字符串转换成整数

    public int strToInt(String str) {
        int res = 0, bndry = Integer.MAX_VALUE / 10;
        int i = 0, sign = 1, length = str.length();
        if(length == 0) {
            return 0;
        }
        while(str.charAt(i) == ' ') {
            if(++i == length) {
                return 0;
            }
        }
        if(str.charAt(i) == '-') {
            sign = -1;
        }
        if(str.charAt(i) == '-' || str.charAt(i) == '+') {
            i++;
        }
        for(int j = i; j < length; j++) {
            if(str.charAt(j) < '0' || str.charAt(j) > '9') {
                break;
            }
            if(res > bndry || res == bndry && str.charAt(j) > '7') {
                return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
            }
            res = res * 10 + (str.charAt(j) - '0');
        }
        return sign * res;
    }
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