leetcode4

4Sum

和3Sum差不多

#coding=utf-8class Solution(object):    def fourSum(self, nums, target):        """        :type nums: List[int]        :type target: int        :rtype: List[List[int]]        """        nums.sort()        result=[]        for i in range(len(nums)-3):            if i==0 or nums[i]>nums[i-1]:                for j in range(i+1,len(nums)-2):                    if j==i+1 or nums[j]>nums[j-1]:                        left=j+1;right=len(nums)-1                        while left<right:                            now=nums[i]+nums[j]+nums[left]+nums[right]                            if now==target:                                result.append([nums[i],nums[j],nums[left],nums[right]])                                left+=1;right-=1                                while left<right and nums[left]==nums[left-1]:                                    left+=1                                while left<right and nums[right]==nums[right+1]:                                    right-=1                            elif now<target:                                left+=1                                while left<right and nums[left]==nums[left-1]:                                    left+=1                            else:                                right-=1                                while left<right and nums[right]==nums[right+1]:                                    right-=1        return result

Remove Nth Node From End of List

C++:

class Solution {public:    ListNode* removeNthFromEnd(ListNode* head, int n) {    if(head==NULL)    {        return NULL;    }    ListNode *Pummy=NULL;    ListNode *p=head;    ListNode *q=head;    for(int i=0;i<n-1;i++)    {        p=p->next;          }    while(p->next)    {        Pummy=q;        q=q->next;        p=p->next;    }    if(Pummy==NULL)    {        head=q->next;        delete q;    }    else    {        Pummy->next=q->next;        delete q;    }    return head;    }};

python:

class Solution(object):    def removeNthFromEnd(self, head, n):        """        :type head: ListNode        :type n: int        :rtype: ListNode        """        dummy=ListNode(0)        dummy.next=head        p1=p2=dummy        for i in range(n):            p1=p1.next        while p1.next:            p1=p1.next            p2=p2.next        p2.next=p2.next.next        return dummy.next

Valid Parentheses

C++:如果当前字符为左半边括号时，则将其压入栈中，如果遇到右半边括号时，若此时栈为空，则直接返回false，如不为空，则取出栈顶元素，若为对应的左半边括号，则继续循环，反之返回false。

class Solution {public:    bool isValid(string s) {        stack<char> parentheses; //堆栈        for(int i=0;i<s.size();i++){            if(s[i]=='(' || s[i]=='[' || s[i]=='{'){                parentheses.push(s[i]);                }            else{                if(parentheses.empty()){                    return false;                }                if(s[i]==')' && parentheses.top()!='(') return false;                if(s[i]==']' && parentheses.top()!='[') return false;                if(s[i]=='}' && parentheses.top()!='{') return false;                parentheses.pop();            }        }        return parentheses.empty();    }};

python：

#coding=utf-8class Solution(object):    def isValid(self, s):        """        :type s: str        :rtype: bool        """        matchDict={'(':')','[':']','{':'}'}        stackList=[]        for i in range(len(s)):            if s[i] not in matchDict.keys() and len(stackList)==0:                return False            elif s[i] in matchDict.keys():                stackList.append(s[i])            elif s[i]==matchDict[stackList[-1]]:#是对应的才能从栈里弹出                stackList.pop()            else:                return False        if len(stackList)==0:            return True        else:            return False

栈(后进先出)

判断是否为空

STACK-EMPTY(S)if S.top==0         return True else return False

压入栈

PUSH(S,x)S.top=S.top+1S[S.top]=x

从栈中弹出

POP(S,x)if STACK-EMPTY(S)    error "underflow"else S.top=S.top-1    return S[S.top+1]

Generate Parentheses

列举出所有合法的括号匹配，使用dfs。如果左括号的数量大于右括号的数量的话，就不能产生合法的括号匹配。

#coding=utf-8class Solution(object):    def helpler(self,l,r,item,res):        print l,r,item        if r<l:            return        if l==0 and r==0:            res.append(item)        if l>0:            self.helpler(l-1,r,item+'(',res)        if r>0:            self.helpler(l,r-1,item+')',res)    def generateParenthesis(self, n):        """        :type n: int        :rtype: List[str]        """        result=[]        self.helpler(n,n,'',result)        return result