I. INTRODUCTION
II. PRELIMINARIES
III. BASIC TECHNIQUES
IV. OVERVIEW OF PPR ALGORITHMS
V. SINGLE-SOURCE PPR ALGORITHMS
VI. SINGLE-TARGET PPR ALGORITHMS
VII. SINGLE-PAIR PPR ALGORITHMS
VIII. TOP-$k$ PPR ALGORITHMS
IX. ALGORITHMS FOR OTHER PPR QUERIES
X. PPR COMPUTATION IN SPECIAL SETTINGS
XI. RESEARCH CHALLENGES AND DIRECTIONS
XII. CONCLUSIONS