一：归并实现
1.递归写法
2.非递归写法，数组模拟递归（STL里源码思路）
二:插入实现
归并递归写法：
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class {
public:
ListNode* merge(ListNode* left, ListNode* right) {
ListNode* result = new ListNode(-1);
ListNode* r = result;
while (left != NULL && right != NULL) {
if (left->val <= right->val) {
r->next = left;
r = r->next;
left = left->next;
} else {
r->next = right;
r = r->next;
right = right->next;
}
}
if (left != NULL) r->next = left;
if (right != NULL) r->next = right;
return result->next;
}
ListNode* sortList(ListNode* head) {
if (head == NULL || head ->next == NULL) return head;
ListNode* pre = NULL;
ListNode* fast = head, *slow = head;
while (fast != NULL && fast->next != NULL) {
pre = slow;
fast = fast->next->next;
slow = slow->next;
}
pre->next = NULL;
ListNode* left = sortList(head);
ListNode* right = sortList(slow);
return merge(left, right);
}
};
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归并非递归写法：
第二种着实很难理解，模拟了很久。敬畏敬畏。大道至简，就是这样哇。
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class {
public:
ListNode* merge(ListNode* l1, ListNode* l2) {
ListNode* result = new ListNode(-1);
ListNode* r = result;
while (l1 && l2) {
if (l1->val <= l2->val) {
r->next = l1;
r = r->next;
l1 = l1->next;
} else {
r->next = l2;
r = r->next;
l2 = l2->next;
}
}
if (l1) {
r->next = l1;
}
if (l2) {
r->next = l2;
}
return result->next;
}
ListNode* table[64] = {NULL};
int fill = 0;
int i = 0;
ListNode* carry;
ListNode* sortList(ListNode* head) {
if (head == NULL || head->next == NULL) return head;
while (head != NULL) {
ListNode* next = head->next;
head->next = carry;
carry = head;
i = 0;
while (i < fill && table[i]) {
table[i] = merge(carry, table[i]);
carry = NULL;
swap(carry, table[i++]);
}
swap(carry, table[i]);
if(i == fill) fill++;
head = next;
}
ListNode* result;
for (int i = 1; i < fill; i++) {
table[i] = merge(table[i], table[i-1]);
}
return table[fill-1];
}
};
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插入排序：
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class {
public:
ListNode* insertionSortList(ListNode* head) {
ListNode* result = new ListNode(INT_MIN);
ListNode* r = result;
ListNode* next;
while (head != NULL) {
next = head->next;
ListNode* r = result;
while (r != NULL && r->next != NULL && r->next->val < head->val) {
r = r->next;
}
head->next = r->next;
r->next = head;
head = next;
}
return result->next;
}
};
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