Dijkstra's 算法

Dijkstra's algorithm, which lets you answer "What's the shortest path to X?" for weighted graphs.

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Dijkstra's 就是在weighted graph 中选出最小值路径

今天完成了每日一题，发现mergesort里面基本的构成就是拆分和merge，这两个你掌握了基本就可以熟练使用了。下面是今天练习的代码：

    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode mid = findMid(head);
        ListNode left = sortList(head);
        ListNode right = sortList(mid);
        return merge(left, right);
    }
    private ListNode findMid(ListNode head) {
        ListNode slow = null, fast = head;
        while (fast != null && fast.next != null) {
            slow = (slow == null) ? head : slow.next;
            fast = fast.next.next;
        }
        ListNode res = slow.next;
        //cut off (split the listnode)
        slow.next = null;
        return res;
    }
    private ListNode merge(ListNode l1, ListNode l2) {
        ListNode prehead = new ListNode();
        ListNode prev = prehead;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                prev.next = l1;
                l1 = l1.next;
            } else {
                prev.next = l2;
                l2 = l2.next;
            }
            prev = prev.next;
        }
        prev.next = (l1 == null) ? l2 : l1;
        return prehead.next;
    }