leetcode2

1.判断字符串是否是回文数

public class Solution {    public boolean isPalindrome(String s) {        if(s ==null || s.length() ==0)            return true;        int start = 0;        int last = s.length()-1;        while(start<last){            while(!isLetterOrNumber(s.charAt(start))&& start<last)                start++;            while(!isLetterOrNumber(s.charAt(last)) && start<last)                last--;            if(Character.toLowerCase(s.charAt(start))!=Character.toLowerCase(s.charAt(last))){                return false;            }                start++;                last--;                }        return true;    }     public boolean isLetterOrNumber(char i){        if(i>='0'&& i<='9' || i>='a'&& i<='z' || i>='A' && i<='Z') return true;        return false;    }}

2.二叉树最大路径和

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    int maxsum = Integer.MIN_VALUE; //初始化最小值    public int maxPathSum(TreeNode root) {        if(root ==null)           return 0;        maxSum(root);        return maxsum;    }    public int maxSum(TreeNode root){        if(root==null)            return 0;        int left = maxSum(root.left);        int right = maxSum(root.right);        int sum = root.val;        if(left>0)            sum+=left;        if(right>0)            sum+=right;        if(maxsum<sum)            maxsum = sum;        return Math.max(left, right) > 0 ? Math.max(left, right) + root.val : root.val;    }}

3获取杨辉三角的第k行

import java.util.*;public class Solution {    public ArrayList<Integer> getRow(int rowIndex) {        ArrayList<Integer> list = new ArrayList<Integer>();        if(rowIndex < 0)            return list;        if(rowIndex ==0){               list.add(1);            return list;            }                  int num[][] = new int[rowIndex+1][rowIndex+1];              for(int i=0;i<rowIndex+1;i++){            num[i][0] = 1;            num[i][i] = 1;            for(int j=1;j<i;j++){                num[i][j] = num[i-1][j] + num[i-1][j-1];            }        }              for(int i=0;i<rowIndex+1;i++){             list.add(num[rowIndex][i]);        }       return list;          }}

4.返回杨辉三角

import java.util.*;public class Solution {    public ArrayList<ArrayList<Integer>> generate(int numRows) {        ArrayList<ArrayList<Integer>> lists = new ArrayList<>();         if(numRows == 0)             return lists;        int num[][] = new int[numRows][numRows];        for(int i=0;i<numRows;i++){            num[i][0] = 1;            num[i][i] = 1;            for(int j=1;j<i;j++){                num[i][j] = num[i-1][j]+ num[i-1][j-1];            }                   }            for(int i=0;i<numRows;i++){            ArrayList<Integer> list = new ArrayList<>();            for(int j=0;j<numRows;j++){                             if(num[i][j]>0)                    list.add(num[i][j]);            }            lists.add(list);        }        return lists;    }}

5.判断是否平衡二叉树

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isBalanced(TreeNode root) {        if(root ==null)            return true;        int left = depth(root.left);        int right = depth(root.right);        if(Math.abs(left-right)>1)            return false;        boolean boolleft = isBalanced(root.left);        boolean boolright = isBalanced(root.right);        return boolleft&&boolright;     }    public int depth(TreeNode root){        if(root ==null)            return 0;        int left = depth(root.left);        int right=depth(root.right);        return (left>right)?(left+1):(right+1);    }}

6，将升序的单项链表转为二叉搜索树

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; next = null; } * } *//** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode sortedListToBST(ListNode head) {        if(head ==null)            return null ;        if(head.next==null)            return new TreeNode(head.val) ;        ListNode mid = head;        ListNode end = head;        ListNode premid = null;        while(end!=null&&end.next!=null){            premid = mid;            mid = mid.next;            end = end.next.next;        }        TreeNode root = new TreeNode(mid.val);        premid.next = null;        root.left = sortedListToBST(head);        root.right = sortedListToBST(mid.next);        return root;    }}

7.排序数组转为二叉搜索树

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode sortedArrayToBST(int[] num) {       int len = num.length;        if(len<=0) return null;        return sorted(num,0,len-1);    }    public TreeNode sorted(int[] num,int start,int end){        if(start>end) return null;        int mid = (start+end)/2 +(start+end)%2;        TreeNode root = new TreeNode(num[mid]);        root.left = sorted(num,start,mid-1);        root.right =sorted(num,mid+1,end);        return root;    }}

8,将二叉树按Z层次输出

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */import java.util.*;public class Solution {    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {        ArrayList<ArrayList<Integer>> lists = new ArrayList<>();          if(root ==null)           return lists;       Queue<TreeNode> q = new LinkedList<TreeNode>();       q.add(root);       int  i=1;       while(!q.isEmpty()){       int n = q.size();                ArrayList<Integer> list = new ArrayList<>();       while(n>0){       TreeNode cur= q.poll();       if(cur.left!=null)       q.add(cur.left);       if(cur.right!=null)       q.add(cur.right);       list.add(cur.val);       n--;             }           i++;       if(i%2!=0){
/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */import java.util.*;public class Solution {    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {        ArrayList<ArrayList<Integer>> lists = new ArrayList<>();        if(root==null)            return lists;        Queue<TreeNode> q =new LinkedList<TreeNode>();        q.add(root);        while(!q.isEmpty()){            ArrayList<Integer> list = new ArrayList<>();            int n = q.size();            while(n>0){                TreeNode cur = q.poll();                if(cur.left!=null)                    q.add(cur.left);                if(cur.right!=null)                    q.add(cur.right);                n--;                list.add(cur.val);            }            lists.add(list);        }        return lists;    }}

for(int j=0;j<list.size()/2;j++){ int temp = list.get(j); list.set(j, list.get(list.size()-j-1)); list.set(list.size()-j-1, temp); } } lists.add(list); }return lists; }}