左右指针

主要解决的是数组、字符串的问题，比如二分查找

左右指针的实际就是 left = 0， right = len(nums) -1通过不断右移left和左移right知道left和right相遇为结束。

1、二分查找

二分查找框架

def binarySearch(int[] nums, int target) {
    int left = 0, right = len(nums) -1;

    while(left <= right) {
        int mid = left + (right - left) / 2;
        if (nums[mid] == target) {
            return mid
        } else if (nums[mid] < target) {
            left = mid + 1
        } else if (nums[mid] > target) {
            right = mid -1
        }
    }
    return -1;
}

2、两数之和

leetcode167 两数之和II

只要数组是有序的情况下，就可以想采用双指针的技巧

class Solution:
    def twoSum(self, numbers: List[int], target: int) -> List[int]:
        left = 0
        right = len(numbers) - 1
        while left <= right:
            count = numbers[left] + numbers[right]
            if count == target:
                return [left + 1,right + 1]
            elif count > target:
                right -= 1
            elif count < target:
                left += 1
        return [-1,-1]

3、反转数组

leetcode344 反转字符串

class Solution:
    def reverseString(self, s: List[str]) -> None:
        """
        Do not return anything, modify s in-place instead.
        """
        left = 0
        right = len(s) -1
        while left < right:
            s[left],s[right] = s[right],s[left]
            left += 1 #右移
            right -= 1#左移