# O(lg(m) + lg(n)) C++ binary search

• First, find the row which might have the target. This is the same as "Search Insert Position": the position minus one is the row number;

Second, find the target in this row.

``````class Solution {
public:
bool searchMatrix(vector<vector<int>>& matrix, int target) {
int n = matrix.size();
int m = matrix[0].size();
int l = 0, r = n - 1, k;

if (target < matrix[0][0]) {           //If the target is less than the first element, abort immediately.
return false;
}

while (l <= r) {
k = l + ((r - l) >> 1);
if (matrix[k][0] > target) {
r = k - 1;
} else if (matrix[k][0] < target) {
l = k + 1;
} else {
return true;
}
}
k = l + ((r - l + 1) >> 1) - 1;         //The inserting position minus one: the row which might have the target
int mid;
l = 0; r = m - 1;
while (l <= r) {
mid = l + ((r - l) >> 1);
if (matrix[k][mid] > target) {
r = mid - 1;
} else if (matrix[k][mid] < target) {
l = mid + 1;
} else {
return true;
}
}
return false;
}
};``````

• can you please explain k=l+((r-l+1)>>1)-1;

• (l+r+1)/2 - 1

• i tried
k= (l+r)>>1;
my code was accepted

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