【leetcode hot 100 240】搜索二维矩阵Ⅱ

解法一：直接查找

class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        for(int i=0; i<matrix.length; i++){
            for(int j=0; j<matrix[0].length; j++){
                if(matrix[i][j]>target){
                    break;
                }
                if(matrix[i][j]==target){
                    return true;
                }
            }
        }
        return false;
    }
}

解法二：二分查找

class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        for (int[] row : matrix) {
            int index = search(row, target);
            if (index >= 0) {
                return true;
            }
        }
        return false;
    }

    public int search(int[] nums, int target) {
        int low = 0, high = nums.length - 1;
        while (low <= high) {
            int mid = (high - low) / 2 + low;
            int num = nums[mid];
            if (num == target) {
                return mid;
            } else if (num > target) {
                high = mid - 1;
            } else {
                low = mid + 1;
            }
        }
        return -1;
    }
}

注意：

• mid = (high - low) / 2 + low

解法三：（Z 字形查找）从右上角开始查找，若matrix[i][j]>target，则列减少；若matrix[i][j]<target，则列增加。

class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        int i=0, j=matrix[0].length-1;
        while(i<matrix.length && j>=0){
            if(matrix[i][j]==target){
                return true;
            }
            if(matrix[i][j]>target){
                j--;
            }
            else{
                i++;
            }
        }
        return false;
    }
}