題目

給定一個包含 n 個整數的排序陣列，找出給定目標值 target 的起始和結束位置。

如果目標值不在陣列中，則返回[-1, -1]

樣例 給出[5, 7, 7, 8, 8, 10]和目標值target=8,返回[3, 4]

分析

二分搜尋法，先二分搜尋左邊界，再二分搜尋右邊界，注意搜尋時邊界的確定 搜尋左邊界的時候，end = mid,保證最後留下的兩個數要麼都等於target要麼第一個小於 搜尋右邊界的時候，start = mid，保證最後留下的兩個數，要麼一個大於，要麼兩個都等於

程式碼

public class Solution {
    /** 
     *@param A : an integer sorted array
     *@param target :  an integer to be inserted
     *return : a list of length 2, [index1, index2]
     */
    public int[] searchRange(int[] A, int target) {
        // write your code here
        if (A.length == 0) {
            return new int[]{-1, -1};
        }
        
        int start, end, mid;
        int[] bound = new int[2]; 
        
        // search for left bound
        start = 0; 
        end = A.length - 1;
        while (start + 1 < end) {
            mid = start + (end - start) / 2;
            if (A[mid] == target) {
                end = mid;
            } else if (A[mid] < target) {
                start = mid;
            } else {
                end = mid;
            }
        }
        if (A[start] == target) {
            bound[0] = start;
        } else if (A[end] == target) {
            bound[0] = end;
        } else {
            bound[0] = bound[1] = -1;
            return bound;
        }
        
        // search for right bound
        start = 0;
        end = A.length - 1;
        while (start + 1 < end) {
            mid = start + (end - start) / 2;
            if (A[mid] == target) {
                start = mid;
            } else if (A[mid] < target) {
                start = mid;
            } else {
                end = mid;
            }
        }
        if (A[end] == target) {
            bound[1] = end;
        } else if (A[start] == target) {
            bound[1] = start;
        } else {
            bound[0] = bound[1] = -1;
            return bound;
        }
        
        return bound;
    }
}
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