Leetcode-Search a 2D Matrix

Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties:

• Integers in each row are sorted from left to right.
• The first integer of each row is greater than the last integer of the previous row.

For example,

Consider the following matrix:

[
  [1,   3,  5,  7],
  [10, 11, 16, 20],
  [23, 30, 34, 50]
]

Given target = 3, return true.

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Analysis:

This is a double binary search problem. First, we use binary search to determine the row in which the target is; Second, we use binary search to search the determined row.

 

Solution:

 

public class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        int xLen = matrix.length;
        if (xLen==0) return false;
        int yLen = matrix[0].length;
        if (yLen==0) return false;

        //Determine row.
        int x = 0;
        int y = xLen-1;
        int k = -1;
        int targetRow = -1;
        while (targetRow==-1){
            if (y<x) return false;

            k = x + (y-x)/2;
            if (target<matrix[k][0]){
                y = k-1;
                continue; 
            } else if (target>matrix[k][yLen-1]){
                x = k+1;
                continue;
            } else {
                targetRow = k;
                continue;
            }
        }

        //Search the target element in the targetRow
        x = 0;
        y = yLen-1;
        k = -1;
        boolean findTarget = false;
        while (!findTarget){
            if (y<x) {
                findTarget = false;
                break;
            }        
         
            k=x+(y-x)/2;
            if (target==matrix[targetRow][k]){
                findTarget = true;
                break;
            } else if (target<matrix[targetRow][k]){
                y = k-1;
                continue;
            } else {
                x = k+1;
                continue;
            }
        }
        return findTarget;
    }
}