L240 Search in 2D Matrix II

Binary Search

Problem:

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 in ascending from left to right.

• Integers in each column are sorted in ascending from top to bottom.

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Example:

Consider the following matrix:

[
  [1,   4,  7, 11, 15],
  [2,   5,  8, 12, 19],
  [3,   6,  9, 16, 22],
  [10, 13, 14, 17, 24],
  [18, 21, 23, 26, 30]
]

Given target = 5, return true.

Given target = 20, return false.

Solution:

• from left bottom corner to check:

  • if larger than the target, move up

  • if smaller than the target, move right

public boolean searchMatrix(int[][] matrix, int target){
    if(matrix == null || matrix.length == 0 || matrix[0] == null || matrix[0].length == 0) return false;
    int row = matrix.length;
    int col = matrix[0].length;
    int r = row - 1;
    int c = 0;
    while(r >= 0 && c < col){
        if(matrix[r][c] == target){
            return true;
        }else if(matrix[r][c] > target){
            r--;
        }else{
            c++;
        }
    }
    return false;
}