**Leetcode 445. Add Two Numbers II

https://leetcode.com/problems/add-two-numbers-ii/description/

递归解：好处是，比自己写Stack快 不知道为啥。。。

class Solution {
public:
	
	int countLen(ListNode* l) {
		int ret = 0;
		while (l) ret++, l = l->next;
		return ret;
	}
	
	ListNode* add(ListNode* l1, ListNode* l2, int &carry) {
		if (!l1 || !l2 ) return NULL;
        
		carry = 0;
		ListNode* tmp = add(l1->next, l2->next, carry);
        
        int v = l1->val + l2->val + carry;
		
		ListNode* now = new ListNode( v % 10 );
		now->next = tmp;
		carry = v/10;
		return now;            
	}
	
	ListNode* addcarry(ListNode* root, int &carry) {
		if (!root) return NULL;

		if (root->next)
			root->next = addcarry(root->next, carry);
		int v = root->val;
		v += carry;
		carry = v/10;
		root->val = v % 10;
		return root;
	}
	
	ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
		if (!l2) return l1;
		if (!l1) return l2;
		int l1len = countLen(l1), l2len = countLen(l2);
		if (l2len > l1len) swap(l1, l2), swap(l2len, l1len);
		if (!l2len) return l1;
		ListNode* newst = l1, *pre = NULL, *ret = NULL;
		int diff = l1len - l2len, carry = 0;
		while (diff--  ) {
            // cout << "newst->" << newst->val << endl;
			pre = newst;
			newst = newst->next;
		}

		if (!pre) {
			ret = add(l1, l2, carry);
			if (carry) {
				ListNode* tmp = new ListNode(carry);
				tmp->next = ret;
				return tmp;
			} else {
				return ret;
			}
		} else {
			ret = add(newst, l2, carry);
			if (carry) {
				pre->next = NULL;
				l1 = addcarry(l1, carry);
				if (carry) {
					ListNode* tmp = new ListNode(carry);
					tmp->next = l1;
					l1 = tmp;
				}   
			}
			pre->next = ret;
			
			ret = l1;
			return ret;
		}
	}
};

自己Stack,代码更清晰

class Solution {
public:
    stack<int> tostack(ListNode* l1) {
        stack<int> sta;
        while (l1) {
            sta.push(l1->val);
            l1 = l1->next;
        }
        return sta;
    }
	
	ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
		if (!l2) return l1;
		if (!l1) return l2;
		stack<int> sta1 = tostack(l1);
        stack<int> sta2 = tostack(l2);
        if (sta2.size() > sta1.size()) {
            swap(l1, l2);
            swap(sta2, sta1);
        }
        int carry = 0;
        ListNode* ret = NULL, *tmp = NULL;
        while (sta2.size()) {
            int v = carry + sta2.top() + sta1.top();
            carry = v/10;
            v %= 10;
            tmp = new ListNode(v);
            tmp->next = ret;
            ret = tmp;
            sta2.pop();
            sta1.pop();
        }
        
        while (sta1.size()) {
            int v = sta1.top() + carry;
            carry = v/10;
            v %= 10;
            tmp = new ListNode(v);
            tmp->next = ret;
            ret = tmp;
            sta1.pop();
        }
        
        if (carry) {
            tmp = new ListNode(carry);
            tmp->next = ret;
            ret = tmp;
        }
        
        return ret;
	}
};