每日收穫(3月17-3月20)

2020年3月17日

1. StringBuilder的append方法可以追加任何基本類型

3月18日

1. 單鏈表 包含 數據 和 next指針
2. 頭節點不包含數據
3. 鏈表插入
   1. 先定位到要插入的節點的前一個元素
   2. 修改前節點的next
   3. 修改插入節點的next
4. 定位函數

定位到當前index對應的節點       

        int i = 0;

        cur = head.next;

        while (cur != null && i < index) {

            cur = cur.next;

            i++;

        }

3月19日

矩陣操作

1. 按照主對角線交換 a[i][j] -- a[j][i]

for(int i = 0; i < matrix.length; I++){

     for (int j = i+1 ; j < matrix.length; j++) {

        int tmp = matrix[i][j];

        matrix[i][j] = matrix[j][i];

        matrix[j][i] = tmp;

      }

}

2. 按照副對角線交換    a[i][j] -- a[n-j][n-i]

for (int i = 0; i < matrix.length; i++){
    for (int j = 0; j < matrix.length-1-i; j++) {
        int tmp = matrix[i][j];
        matrix[i][j] = matrix[n-j][n-i];
        matrix[n-j][n-i] = tmp;
    }
}

3. 按照行上下顛倒

for (int i= 0; i < n/2; i++) {
    for( int j = 0; j < matrix.length; j++) {
        int tmp = matrix[i][j];
        matrix[i][j] = matrix[n-1-i][j];
        matrix[n-1-i][j] = tmp;
    }
}

4. 順時針旋轉90度 (先上下顛倒, 然後按照主對角線交換)

for (int i =0 ;i < n/2; i++){
    for (int j = 0; j < a.length; j++){
        int tmp = a[i][j];
        a[i][j] = a[n-1-i][j];
        a[n-i-1][j] = tmp;
    }
}

for (int i = 0; i < a.length; i++){
    for (int j = i+1; j < a.length; j++){
        int tmp = a[i][j];
        a[i][j] = a[j][i];
        a[j][i] = tmp;
    }
}

5. 逆時針旋轉90度(先上下顛倒, 然後按照副對角線交換)

for (int i = 0; i < n/2; i++) {
    for (int j = 0; j < n; j++){
        int tmp = a[i][j];
        a[i][j] = a[n-1-i][j];
        a[n-i-1][j] = tmp;
    }
}

for (int i = 0; i < n-1; i++){
    for (int j = 0; j < n-i-1; j++;){
        int tmp = a[i][j];
        a[i][j] = a[n-j][n-i];
        a[n-j][n-i] = tmp;
    }
}

3月20日

查找倒數第K個鏈表節點

    public ListNode FindKthToTail(ListNode head,int k) {
        ListNode fast = head, slow = head;
        int i = 0;
        while (fast != null) {
            if (i >= k) {
                slow = slow.next;
            }
            fast = fast.next;
            i++;
        }
        return i < k ? null : slow;
    }

鏈表A+B

public ListNode plusAB(ListNode a, ListNode b) {
        // write code here
        if (a == null)return b;
        if (b == null)return a;
        ListNode head = new ListNode(-1);
        ListNode cur = head;
        int sum = 0;
        while (a != null || b != null || sum != 0) {
            if (a != null) {
                sum += a.val;
                a = a.next;
            }
            if (b != null) {
                sum += b.val;
                b = b.next;
            }
            cur.next = new ListNode(sum%10);
            cur = cur.next;
            sum /= 10;
        }
        return head.next;
    }

用2個棧實現隊列

import java.util.Stack;

public class Solution {
    Stack<Integer> stack1 = new Stack<Integer>();
    Stack<Integer> stack2 = new Stack<Integer>();
    
    public void push(int node) {
        stack1.push(node);
    }
    
    public int pop() {
        if (stack1.empty() && stack2.empty()) {
            throw new RuntimeException("Queue is null");
        }
        if (stack2.empty()) {
            while (!stack1.empty()) {
                stack2.push(stack1.pop());
            }
        }
        return stack2.pop();
    
    }
}