一、链表是否有环

var hasCycle = function(head) {
    let fast, slow;
    fast = slow = head
    while(fast !== null && fast.next !== null) {
        fast = fast.next.next
        slow = slow.next
        if(fast === slow) {
            return true
        }
    }
    return false
}

二、环的起点

var detectCycle = function(head) {
    let fast, slow;
    fast = slow = head
    while(fast !== null && fast.next !== null) {
        fast = fast.next.next
        slow = slow.next
        if(fast === slow) break;     
    }
    if(fast === null || fast.next === null) {
        return null
    }
    slow = head
    while(slow !== fast) {
        fast = fast.next
        slow = slow.next
    }
    return slow
}

三、链表中间节点

var middleNode = function(head) {
    let fast, slow;
    fast = slow = head;
    while (fast != null && fast.next != null) {
        fast = fast.next.next;
        slow = slow.next;
    }
    // slow 就在中间位置
    return slow;
};

四、链表倒数第K个元素

var getKthFromEnd = function(head, k) {
    let slow, fast;
    slow = fast = head;
    while (k-- > 0) 
        fast = fast.next;
    while (fast != null) {
        slow = slow.next;
        fast = fast.next;
    }
    return slow;
};

五、二分查找

var search = function(nums, target) {
    if (nums.length === 0) return -1;
    let left = 0, right = nums.length - 1;
    while (left <= right) {
        let mid = Math.floor(left + (right - left) / 2);
        if (nums[mid] < target) {
            left = mid + 1;
        } else if (nums[mid] > target) {
            right = mid - 1;
        } else if (nums[mid] === target) {
            // 直接返回
            return mid;
        }
    }
    // 直接返回
    return -1;
};

六、两数之和

var twoSum = function(nums, target) {
    let left = 0, right = nums.length - 1;
    while (left < right) {
        let sum = nums[left] + nums[right];
        if (sum === target) {
            // 题目要求的索引是从 1 开始的
            return [left + 1, right + 1];
        } else if (sum < target) {
            left++; // 让 sum 大一点
        } else if (sum > target) {
            right--; // 让 sum 小一点
        }
    }
    return [-1, -1];
};

七、反转数组

var reverseString = function(s) {
    let left = 0;
    let right = s.length - 1;
    while (left < right) {
        // swap(s[left], s[right])
        let temp = s[left];
        s[left] = s[right];
        s[right] = temp;
        left++; right--;
    }
};