【leetcode】34. Search for a Range

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于 2015-10-07 14:22:52 发布

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分类专栏：【leetcode】文章标签： binary search leetcode search for a range

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二分查找，查找有序可重复数列的最左和最右出现的数的下标。

可以参考，通过二分的写法理解循环不变式：http://www.cnblogs.com/wuyuegb2312/archive/2013/05/26/3090369.html

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        int left = -1, right = -1;
        int len = nums.size();
        if(len == 0) {
            return vector<int> {left, right};
        }
        // left index
        int mid;
        int lf = 0, rt = len-1;
        while(lf < rt) {
            mid = lf + (rt-lf) / 2;
            if(nums[mid] >= target) rt = mid;
            else lf = mid + 1;
        }
        if(nums[lf] == target) left = lf;
        // right index
        lf = 0; rt = len-1;
        bool flag = false;
        while(lf < rt - 1) {
            mid = lf + (rt - lf) / 2;
            if(nums[mid] <= target) lf = mid;
            else rt = mid;
        }
        if(nums[rt] == target) right = rt;
        else if(nums[lf] == target) right = lf;
        
        
        return vector<int>{left, right};
    }
};