explanation
// 两个数字位数一样吗？
// 位数不一样也没有关系，因为是reverse order， head是低位，无论如何都要从低位开始加，进位到后面的node
method 1: 三个while
O(n + m)
或者
O(max(m, n)) while iterator O(max(m, n))次
method 2: 一个while O(n + m)
或者
O(max(m, n))
code
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class {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode dummy = new ListNode(0);
ListNode prev = dummy;
int carry = 0;
while (l1 != null && l2 != null) {
int sum = l1.val + l2.val + carry;
int curt = sum % 10;
carry = sum / 10;
ListNode newNode = new ListNode(curt);
prev.next = newNode;
prev = prev.next;
l1 = l1.next;
l2 = l2.next;
}
while ( l1 != null) {
int sum = l1.val + carry;
int curt = sum % 10;
carry = sum / 10;
ListNode newNode = new ListNode(curt);
prev.next = newNode;
prev = prev.next;
l1 = l1.next;
}
while ( l2 != null) {
int sum = l2.val + carry;
int curt = sum % 10;
carry = sum / 10;
ListNode newNode = new ListNode(curt);
prev.next = newNode;
prev = prev.next;
l2 = l2.next;
}
if (carry > 0) {
ListNode newNode = new ListNode(carry);
prev.next = newNode;
}
return dummy.next;
}
}
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class {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode dummy = new ListNode(0);
ListNode prev = dummy;
int carry = 0;
while (l1 != null || l2 != null || carry != 0) {
int x = (l1 == null) ? 0 : l1.val;
int y = (l2 == null) ? 0 : l2.val;
int sum = x + y + carry;
int curt = sum % 10;
carry = sum / 10;
ListNode newNode = new ListNode(curt);
prev.next = newNode;
prev = prev.next;
if (l1 != null) {
l1 = l1.next;
}
if (l2 != null) {
l2 = l2.next;
}
}
return dummy.next;
}
}
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