leetcode心得
#74. Search a 2D Matrix

• 用兩次binary search
• 用一次binary search

class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
    //using two binary search
    //check errors
    if(matrix==null||matrix.length==0){
        return false;
    }
    if(matrix[0]==null||matrix[0].length==0){
        return false;
    }
    int row = matrix.length;
    int col = matrix[0].length;
    //通過每一行第一個元素確定在哪一行
    int start=0,end=row-1;
    while(start+1<end){
        int mid=start+(end-start)/2;
        if (matrix[mid][0]==target){
            return true;
        }
        if(matrix[mid][0]>target){
            end=mid;
        }
       else {
            start=mid;
        }
    }
    if(matrix[end][0]<=target){//end必須放在前面，因為start和end可能同時滿足
        row=end;//在這裡需要找到目標所在的行
    }
    else if(matrix[start][0]<=target){ //如果這裡改成if，就不正確
        row=start;
    }
    else{
        return false;
    }
    //else if(matrix[start][0]<target&&matrix[start+1][0]>target){ 直接順序執行，因為找出了所在行，所以不用判斷，判斷的話會出現會出現越界
    int hi=0, lo=col-1;
    while(hi+1<lo){
            int p = hi+(lo-hi)/2;
            if (matrix[row][p]==target){
                return true;}
            if (matrix[row][p]>target){
                lo=p;
            }
            else {
                hi=p;
            }
    }
    if (matrix[row][hi]==target){
            return true;
        }
    else if (matrix[row][lo]==target){
            return true;
        }
    return false;
    }
}
 

用一次二分搜尋
public boolean searchMatrix(int[][] matrix, int target) {
        // write your code here
        if(matrix == null || matrix.length == 0){
            return false;
        }
        
        if(matrix[0] == null || matrix[0].length == 0){
            return false;
        }
        
        int row = matrix.length;
        int column = matrix[0].length;
        
        int start = 0, end = row * column - 1;
        while(start <= end){
            int mid = start + (end - start) / 2;
            int number = matrix[mid / column][mid % column];//矩陣的正中間點
            if(number == target){
                return true;
            }else if(number > target){
                end = mid - 1;
            }else{
                start = mid + 1;
            }
        }
        
        return false;
        
    }
}