双指针是在有序数组或链表上常用的优化技巧，把 O(n²) 的问题优化到 O(n)。

一、移动零

public void moveZeroes(int[] nums) {
    int nonZero = 0;
    for (int i = 0; i < nums.length; i++) {
        if (nums[i] != 0) {
            int temp = nums[i];
            nums[i] = nums[nonZero];
            nums[nonZero] = temp;
            nonZero++;
        }
    }
}

二、盛最多水的容器

public int maxArea(int[] height) {
    int max = 0, left = 0, right = height.length - 1;
    while (left < right) {
        int area = Math.min(height[left], height[right]) * (right - left);
        max = Math.max(max, area);
        if (height[left] < height[right]) {
            left++;
        } else {
            right--;
        }
    }
    return max;
}

三、环形链表

public boolean hasCycle(ListNode head) {
    ListNode slow = head, fast = head;
    while (fast != null && fast.next != null) {
        slow = slow.next;
        fast = fast.next.next;
        if (slow == fast) return true;
    }
    return false;
}

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